Gmail / Google Workspace error guide

Gmail / 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.

Quick summary

Primary code
523 5.7.10
Provider coverage
Gmail / Google Workspace
Primary action
Fix sender setup

How to handle 523 5.7.10

Match the provider, SMTP stage, endpoint, and diagnostic wording first, then use the checks below to decide whether to retry, suppress the recipient, or fix sender setup.

Gmail 523 5.7.10

SMTP protocol

Fix sender setup

SMTP protocol means the receiving provider rejected the message for a provider-specific reason. Use the diagnostic text before deciding on retries or suppression.

Decision

Do not retry unchanged. Fix the sender setup first.

Confidence: high. Recipient validity: unknown.

Match this reply

SMTP stage
STARTTLS
Endpoint
Inbound MX
pipelineSTARTTLS

Start here

  1. 1

    Review the SMTP client behavior and command sequence.

  2. 2

    Fix protocol violations such as invalid pipelining before retrying.

Confirm the cause

  1. Match provider, SMTP stage, endpoint, reply code, enhanced code, and diagnostic text.
  2. Run the sender, recipient, DNS, and content checks that match the diagnostic wording.
  3. Test with one recipient before retrying bulk traffic.

App handling

  • Treat this as a sender setup incident, not a bad recipient.
  • Hold similar mail for the same sender domain or provider until the fix is verified.
  • Retry after a fresh single-recipient test succeeds.

Avoid

  • Do not retry unchanged in a loop.
  • Do not suppress the recipient unless the text explicitly says the mailbox is invalid.
  • Do not treat a permanent SMTP code as proof that the address is bad; policy and routing failures can also be permanent.
Common causes
  • The provider-specific diagnostic text matches this class of failure.
  • The same SMTP code may have another meaning when the provider, stage, or endpoint differs.
  • Sender setup, recipient status, and provider policy should all be checked before taking irreversible action.
Developer details
Fields to log
raw SMTP replyremote MX or hostSMTP stagereply codeenhanced codeprovidermatched rule idrecipientsending domaindecisiontimestamp
Classifier evidence
Specific cause
invalid pipelining after starttls
Evidence
official
support.google.com source 1

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