451 4.3.0: several recipient domains in one transaction
Your server put recipients at more than one domain into a single SMTP transaction with Gmail, which Gmail does not support, so it asks for a retry.
- Gmail
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 451 4.3.0
- Provider
- Gmail and Google Workspace
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
451 4.3.0 Multiple destination domains per transaction is unsupported. Please try again.
What it means
In one SMTP transaction the sender gives one MAIL FROM, one or more RCPT TO commands, and then the message. Gmail does not support recipients at different destination domains in the same transaction. When a transaction mixes them, Gmail answers with this temporary reply and asks for the message to be tried again. Most mail servers already group recipients by domain, so this tends to come from custom sending code, or from a relay or gateway that batches recipients across domains to save connections. Google's page refers to RFC 5321 for how transactions are built.
Common causes
- Custom sending code that adds recipients at several domains to one transaction.
- A relay or gateway set up to batch recipients across domains on one connection.
How to fix it
- 01Send one transaction per recipient domain: a separate MAIL FROM, RCPT TO and DATA for each domain.
- 02If you run a mail server, check its settings for grouping recipients by destination domain.
- 03Let queued messages retry after the change, since Gmail asks for the deferred recipients to be tried again.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Gmail SMTP errors and codes (Google Workspace Admin Help)knowledge.workspace.google.com
Related codes
- 452 4.5.3Recipient needs its own transaction.The receiving domain's policy settings mean Gmail cannot take this recipient in the same transaction as the others, so it asks you to retry it separately.
- 451Local error in processing.The receiving server hit a problem on its own side while handling the message and stopped. The reply is temporary, so the sending server will try again.
- 5.3.0Receiving system problem, unspecified.The receiving server normally accepts mail, but a problem on that system itself, rather than with the mailbox or the message, stopped this delivery.
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