Can I do anything about a 451 from someone else’s server?

Mostly no. Your server retries on its own; if the problem lasts, the recipient’s admin has to fix it, so give them the full error and timestamp.

Why does Microsoft send 451 4.7.500?

That range is Microsoft 365 throttling unfamiliar or suspicious senders; see the dedicated 4.7.500 page for what to change.

SMTP 451: local error in processing

451Requested action aborted: local error in processingTemporary: the sender retries

Example

451 4.3.0 <user@example.com>: Temporary lookup failure

What it means

RFC 5321 defines 451 as "Requested action aborted: local error in processing". The word local refers to the server sending the reply: something went wrong on its side, such as a database, DNS, content scanner, or spool problem, and it chose to defer rather than lose the message.

It can answer RCPT TO (often a failed recipient lookup) or the end of DATA (often a crashed or timed out filter). The enhanced code narrows it down: 4.3.0 is a generic system issue, 4.4.3 is a directory or DNS failure, 4.7.1 is usually a policy deferral, and 4.3.2 means the system is not accepting messages.

Your server retries automatically. Unlike 450, which is about the mailbox, and 452, which is about storage, 451 is a catch all for a temporary fault in the receiving system. Microsoft 365 also uses 451 4.7.500 to 4.7.699 ranges for its own throttling, which have their own pages.

Likely causes

The receiver’s recipient lookup (LDAP, SQL, or a directory sync) failed or timed out.
A content filter, antivirus scanner, or milter on the receiving side crashed or took too long.
The receiver could not resolve DNS for your domain or for its own routing data.
A provider side throttle, such as Microsoft 365 451 4.7.500 style deferrals for new or suspicious senders.
Your own relay returns 451 to your application because its upstream smarthost or queue is unhealthy.

How to fix it

Note which host returned the 451 and at which stage (RCPT or after DATA). If it is your own relay, check its logs first.
Let the queue retry for a few hours. Many 451 replies come from short outages on the receiving side.
If the text mentions a lookup or DNS failure, check that your domain’s nameservers answer reliably and that DNSSEC, if enabled, validates.
If it lasts more than a day, send the recipient’s admin the full reply, the time, and your sending IP; only they can fix their filter or directory.

Checks that apply

Your domain or sending IP (fills in the checks below)

DNS Server Audit Checks your nameservers for timeouts and inconsistencies that make receivers fail lookups and answer 451.
DNSKEY Lookup Shows whether your DNSSEC keys validate, since a broken chain makes validating receivers defer with temporary lookup errors.
SMTP Diagnostics Tests the receiving MX directly so you can see whether it is healthy or answering every session with a temporary error.

FAQ

Can I do anything about a 451 from someone else’s server?: Mostly no. Your server retries on its own; if the problem lasts, the recipient’s admin has to fix it, so give them the full error and timestamp.
Why does Microsoft send 451 4.7.500?: That range is Microsoft 365 throttling unfamiliar or suspicious senders; see the dedicated 4.7.500 page for what to change.

All 191 bounce codes