SMTP 471: mail rejected because of a problem on the receiving server

471 is a 4xx temporary failure most commonly generated by Barracuda Email Security Gateway appliances and some older on-premises Exchange transport agents, typically with the enhanced code 4.7.0 or 4.5.0 attached. It signals that the receiving side hit an internal processing error - a local anti-spam engine timing out, a transport rule misfiring, or a connection-handling fault - while trying to evaluate your message. It is not part of the RFC 5321 base code table, but it is a real wire reply from specific, identifiable software.

Who is at fault

The fault is on the receiving server. The failure happens inside the recipient's filtering or transport layer before it has made any judgement about your content or sender reputation. Retrying is the correct response, and retrying usually works because the fault is typically transient (a filter engine restart, a momentary overload). Barracuda appliances in particular are prone to brief internal restarts when their filtering

engine's definitions update, which can produce a short burst of 471 replies across many messages

in a tight window followed by entirely normal behaviour minutes later. If you see several 471s

clustered tightly in time from the same domain and then nothing for hours afterward, that pattern

is consistent with exactly this kind of definition-update restart rather than a persistent

capacity problem, and no corrective action beyond the normal retry is needed.

The exchange

220 mail.example.org ESMTP
EHLO mail.yourdomain.com
250-mail.example.org
250 SIZE 10485760
MAIL FROM:<sender@yourdomain.com>
250 2.1.0 Sender OK
RCPT TO:<recipient@example.org>
250 2.1.5 Recipient OK
DATA
354 Enter mail, end with "." on a line by itself
471 4.7.0 Message rejected due to local policy (BRTS filter error)

How Gmail, Microsoft 365 and Yahoo word the same category of condition

None of the big three use "471" literally - it is characteristic of Barracuda-protected domains, which are common among mid-size businesses running their own mail security appliance rather than a cloud mailbox provider.

Diagnostic commands

Confirm the receiving side is Barracuda-protected and that the condition is transient:

$ dig +short MX example.org
mail.example.org.

$ nslookup -type=txt example.org | grep -i barracuda

$ swaks --to recipient@example.org --from sender@yourdomain.com --server mail.example.org
<~  471 4.7.0 Message rejected due to local policy (BRTS filter error)

$ sleep 600 && swaks --to recipient@example.org --from sender@yourdomain.com --server mail.example.org
<~  250 2.0.0 Ok: queued as 4F3A218

A second attempt after ten to fifteen minutes succeeding confirms the fault was transient on their end, not a persistent block tied to your sending IP.

The ordered fix

1. Confirm your own queue is retrying, not dropping the message after the first 4xx.
2. Wait through at least two retry cycles (commonly 15 minutes to a few hours apart) before treating it as stuck.
3. If it recurs for the same domain across multiple sends, check whether your sending IP or domain shows up in Barracuda's reputation system via their own lookup tool.
4. If it clears for other recipients at the same domain but not one specific address, suspect a per-mailbox rule rather than a gateway-wide fault.
5. Contact the recipient through another channel only after 24 hours of consistent failure, citing the exact reply text.

What not to do

• Do not assume this means you are blocklisted. 471 is a processing fault, not a reputation verdict, and checking blocklists first wastes time that should go to simple retries.
• Do not change your SPF/DKIM alignment in response. This error occurs after authentication has already been evaluated (or before it is even reached), so it has nothing to do with alignment.
• Do not suppress the recipient address. No permanent judgement about deliverability to that address was made.
• Do not hammer retries faster than your MTA's normal backoff. Rapid re-sends into a gateway that is already erroring internally can make the appliance's state worse, not better.

Distinguishing this from a genuine block

Barracuda-protected domains sometimes escalate from a transient 471 to an outright policy block if

the underlying trigger (a local filter engine repeatedly failing, or a sending pattern the appliance

flags) persists. The distinguishing signal is whether the reply text changes between attempts - a

persistent, identically worded 471 across many hours suggests a stuck appliance process rather than

an escalating policy decision, while a shift toward wording that mentions reputation, blocklists, or

explicit denial is a sign the condition has moved from "processing fault" to "deliberate rejection,"

and warrants checking your own sending reputation at that point instead of just waiting it out.

MailPilot tracks deferrals like this separately from true reputation events in its per-mailbox daily scoring, so a Barracuda-side hiccup at one recipient domain does not get folded into your overall sender health the way a real complaint or blocklist hit would.