SMTP bounce diagnostics
SMTP error codes, decoded by provider.
A numeric SMTP code is only part of the answer. Compare Gmail, Microsoft, Yahoo/AOL, Apple iCloud, and Apple Private Relay diagnostics before you retry, suppress, or change sender setup.
Provider comes first. Then endpoint, SMTP stage, reply code, enhanced code, provider token, and diagnostic text.
That is why one 550 5.1.1 response can be an invalid mailbox, while another can be a Private Relay sender authorization problem.
Provider-specific
The guides separate Gmail, Microsoft, Yahoo/AOL, Apple iCloud, and Apple Private Relay behavior instead of treating every code as universal.
Action-oriented
Each guide calls out whether to retry later, suppress the recipient, fix sender setup, or investigate the bounce further.
Full-reply matching
The lookup tool can use the provider token and diagnostic text, not only the first three SMTP digits.
Canonical code pages
Shared SMTP codes
These codes are common enough across providers that one canonical page with provider sections is clearer than several competing pages.
550 5.1.1SMTP 550 5.1.1: provider meanings and fixes
SMTP 550 5.1.1 can mean invalid recipient, unavailable mailbox, or provider-specific authorization issue. Compare Gmail, Microsoft, and Apple handling before retrying or suppressing.
550 5.7.1SMTP 550 5.7.1: provider meanings and fixes
SMTP 550 5.7.1 can mean policy rejection, authorization failure, blocked content, or rejected sender. Compare Gmail, Microsoft, and Apple handling before retrying or suppressing.
421 4.7.0SMTP 421 4.7.0: provider meanings and fixes
SMTP 421 4.7.0 can mean temporary deferral caused by rate limits, sender reputation, or policy checks. Compare Gmail and Yahoo/AOL handling before retrying or suppressing.
552 5.2.2SMTP 552 5.2.2: provider meanings and fixes
SMTP 552 5.2.2 can mean recipient mailbox quota, storage, or account capacity issue. Compare Gmail and Apple handling before retrying or suppressing.
550 5.4.1SMTP 550 5.4.1: Microsoft 365 recipient lookup or relay error
In Exchange Online, 550 5.4.1 usually means Directory-Based Edge Blocking rejected the recipient, or a connector/relay route is denied. Check the diagnostic text before suppressing the address or retrying.
Provider pages
Provider-specific SMTP fingerprints
These pages target codes and tokens that are strongly tied to a provider, such as Gmail authentication failures, Microsoft NDR codes, Yahoo throttles, and Apple Private Relay authorization.
Gmail / Google Workspace
18 guides421 4.7.26Gmail / Google Workspace SMTP 421 4.7.26: Spf Or Dkim Failed Temporary
Gmail / Google Workspace SMTP 421 4.7.26 usually means sender authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
421 4.7.27Gmail / Google Workspace SMTP 421 4.7.27: Spf Failed Bulk Sender
Gmail / Google Workspace SMTP 421 4.7.27 usually means sender authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
421 4.7.28Gmail / Google Workspace SMTP 421 4.7.28: Gmail Bulk Rate Or Reputation
Gmail / Google Workspace SMTP 421 4.7.28 usually means rate or reputation. Learn whether to retry, suppress the recipient, or fix sender setup.
421 4.7.29Gmail / Google Workspace SMTP 421 4.7.29: Tls Required Bulk Sender
Gmail / Google Workspace SMTP 421 4.7.29 usually means transport security. Learn whether to retry, suppress the recipient, or fix sender setup.
421 4.7.30Gmail / Google Workspace SMTP 421 4.7.30: Dkim Failed Bulk Sender
Gmail / Google Workspace SMTP 421 4.7.30 usually means sender authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
421 5.7.32Gmail / Google Workspace SMTP 421 5.7.32: From Domain Alignment Temporary
Gmail / Google Workspace SMTP 421 5.7.32 usually means sender authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
421 4.7.40Gmail / Google Workspace SMTP 421 4.7.40: Dmarc Missing Or Invalid Bulk Sender
Gmail / Google Workspace SMTP 421 4.7.40 usually means sender authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
450 4.2.1Gmail / Google Workspace SMTP 450 4.2.1: Recipient Or Relay Rate Limit
Gmail / Google Workspace SMTP 450 4.2.1 usually means rate limit. Learn whether to retry, suppress the recipient, or fix sender setup.
451 4.7.23Gmail / Google Workspace SMTP 451 4.7.23: Ptr Missing Or Mismatch
Gmail / Google Workspace SMTP 451 4.7.23 usually means dns configuration. Learn whether to retry, suppress the recipient, or fix sender setup.
451 4.7.24Gmail / Google Workspace SMTP 451 4.7.24: Spf Record Suspicious
Gmail / Google Workspace SMTP 451 4.7.24 usually means sender authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
451 4.7.26Gmail / Google Workspace SMTP 451 4.7.26: Temporary Dmarc Dns Failure
Gmail / Google Workspace SMTP 451 4.7.26 usually means dns configuration. Learn whether to retry, suppress the recipient, or fix sender setup.
454 4.7.0Gmail / Google Workspace SMTP 454 4.7.0: Temporary Auth Failure
Gmail / Google Workspace SMTP 454 4.7.0 usually means smtp authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.2.1Gmail / Google Workspace SMTP 550 5.2.1: Recipient Inactive Or Disabled
Gmail / Google Workspace SMTP 550 5.2.1 usually means recipient issue. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.26Gmail / Google Workspace SMTP 550 5.7.26: Spf Dkim Or Dmarc Failure
Gmail / Google Workspace SMTP 550 5.7.26 usually means sender authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
552 5.7.0Gmail / Google Workspace SMTP 552 5.7.0: Blocked Attachment Or Security Content
Gmail / Google Workspace SMTP 552 5.7.0 usually means message content. Learn whether to retry, suppress the recipient, or fix sender setup.
552 5.3.4Gmail / Google Workspace SMTP 552 5.3.4: Message Or Header Size Limit
Gmail / Google Workspace SMTP 552 5.3.4 usually means message size. Learn whether to retry, suppress the recipient, or fix sender setup.
554 5.4.6Gmail / Google Workspace SMTP 554 5.4.6: Routing Loop
Gmail / Google Workspace SMTP 554 5.4.6 usually means routing. Learn whether to retry, suppress the recipient, or fix sender setup.
523 5.7.10Gmail / Google Workspace SMTP 523 5.7.10: Invalid Pipelining After Starttls
Gmail / Google Workspace SMTP 523 5.7.10 usually means smtp protocol. Learn whether to retry, suppress the recipient, or fix sender setup.
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online
31 guides432 4.3.2Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 432 4.3.2: Recipient Thread Limit
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 432 4.3.2 usually means rate limit. Learn whether to retry, suppress the recipient, or fix sender setup.
450 4.4.317Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 450 4.4.317: Tls Untrusted Root
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 450 4.4.317 usually means transport security. Learn whether to retry, suppress the recipient, or fix sender setup.
451 4.7.0Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 451 4.7.0: Temporary Auth Or Proxy Failure
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 451 4.7.0 usually means smtp authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
451 4.7.500.699.suspiciousMicrosoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 451 4.7.500.699.suspicious: Temporary Suspicious Activity Restriction
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 451 4.7.500.699.suspicious usually means temporary reputation or policy. Learn whether to retry, suppress the recipient, or fix sender setup.
451 4.7.850.899.ip.restrictionMicrosoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 451 4.7.850.899.ip.restriction: Temporary Ip Restriction
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 451 4.7.850.899.ip.restriction usually means temporary reputation or policy. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.1.8Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.1.8: Outbound Sender Blocked
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.1.8 usually means sender account. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.1.10Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.1.10: Invalid Recipient
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.1.10 usually means recipient issue. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.1.20Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.1.20: Multiple From Without Sender
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.1.20 usually means message format. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.2.2Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.2.2: Exchange Contextual 5 2 2
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.2.2 usually means quota or rate limit. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.4.14Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.4.14: Routing Loop
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.4.14 usually means routing. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.4.300Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.4.300: Delivery Expired
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.4.300 usually means routing. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.5.0Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.5.0: Mailbox Unavailable
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.5.0 usually means recipient issue. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.6.11Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.6.11: Bare Lf
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.6.11 usually means message format. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.23Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.23: Spf Policy Violation
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.23 usually means sender authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.25Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.25: Ipv6 Reverse Dns Missing
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.25 usually means dns configuration. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.57Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.57: Smtp Client Not Authenticated
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.57 usually means smtp authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.64Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.64: Tenant Attribution Failed
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.64 usually means connector configuration. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.124Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.124: Group Sender Not Allowed
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.124 usually means recipient policy. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.133Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.133: External Sender Not Allowed For Group
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.133 usually means recipient policy. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.232Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.232: Trial Tenant External Recipient Limit
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.232 usually means sender quota. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.233Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.233: Tenant External Recipient Limit
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.233 usually means sender quota. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.509Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.509: Dmarc Reject
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.509 usually means sender authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.511Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.511: Sending Ip Banned
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.511 usually means reputation or policy. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.515Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.515: Outlook Consumer High Volume Auth Requirement
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.515 usually means sender authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.606.649.blocklistMicrosoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.606.649.blocklist: Microsoft Ip Blocklist
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.606.649.blocklist usually means reputation or policy. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.705Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.705: Tenant Suspicious Traffic Threshold
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.705 usually means sender policy. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.708Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.708: Source Traffic Not Accepted
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.708 usually means reputation or policy. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.750Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.750: Unregistered Sending Domain
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 550 5.7.750 usually means sender configuration. Learn whether to retry, suppress the recipient, or fix sender setup.
535 5.7.142Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 535 5.7.142: Oauth Token Expiring
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 535 5.7.142 usually means smtp authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
535 5.7.143Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 535 5.7.143: Oauth Token Expired
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 535 5.7.143 usually means smtp authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
535 5.7.144Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 535 5.7.144: Oauth Invalid Api Permissions
Microsoft Outlook / Hotmail / Live / MSN / Exchange Online SMTP 535 5.7.144 usually means smtp authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
Yahoo Mail / AOL
6 guides451Yahoo Mail / AOL SMTP 451: Yahoo Rl01
Yahoo Mail / AOL SMTP 451 usually means temporary reputation or volume throttle. Learn whether to retry, suppress the recipient, or fix sender setup.
451Yahoo Mail / AOL SMTP 451: Resources Unavailable
Yahoo Mail / AOL SMTP 451 usually means temporary server failure. Learn whether to retry, suppress the recipient, or fix sender setup.
451Yahoo Mail / AOL SMTP 451: Temporary Dns Or Lookup Failure
Yahoo Mail / AOL SMTP 451 usually means temporary server or dns failure. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.9Yahoo Mail / AOL SMTP 550 5.7.9: Spf Or Dkim Not Satisfied
Yahoo Mail / AOL SMTP 550 5.7.9 usually means sender authentication. Learn whether to retry, suppress the recipient, or fix sender setup.
554Yahoo Mail / AOL SMTP 554: Invalid Recipient
Yahoo Mail / AOL SMTP 554 usually means recipient issue. Learn whether to retry, suppress the recipient, or fix sender setup.
554Yahoo Mail / AOL SMTP 554: Rfc Or Domain Policy Rejection
Yahoo Mail / AOL SMTP 554 usually means message format or sender domain. Learn whether to retry, suppress the recipient, or fix sender setup.
Apple iCloud / me.com / mac.com / Private Relay
3 guides421 4.7.1Apple iCloud / me.com / mac.com / Private Relay SMTP 421 4.7.1: Icloud Ab02
Apple iCloud / me.com / mac.com / Private Relay SMTP 421 4.7.1 usually means temporary policy or reputation. Learn whether to retry, suppress the recipient, or fix sender setup.
550 5.7.0Apple iCloud / me.com / mac.com / Private Relay SMTP 550 5.7.0: Sending Ip Or Reputation Block
Apple iCloud / me.com / mac.com / Private Relay SMTP 550 5.7.0 usually means reputation or policy. Learn whether to retry, suppress the recipient, or fix sender setup.
550/554 5.7.1 CS01Apple iCloud / me.com / mac.com / Private Relay SMTP 550/554 5.7.1 CS01: Icloud Local Policy Cs01
Apple iCloud / me.com / mac.com / Private Relay SMTP 550/554 5.7.1 CS01 usually means policy rejection. Learn whether to retry, suppress the recipient, or fix sender setup.
Cause groups
Common SMTP rejection causes
The lookup data is also grouped by operational cause so it is easier to map a bounce to a sender, recipient, DNS, routing, or policy fix.
Need reliable SMTP sending?
Sidemail gives SaaS and AI teams practical email delivery infrastructure, SMTP relay, API sending, logs, and tools to debug problems quickly.
Start free trial