topic Re: The '451 4.3.2 Internal server error' in Email

The '451 4.3.2 Internal server error'

keeka — Wed, 30 Sep 2026 12:19:50 GMT

I am sending my own domain (not PN hosted) email via relay.plus.net. I just encountered the refusal to relay today having not seen it for years, if ever.

I'm aware of advice to set smtp auth when users reported this message in the recent past. What I would like to know is: Is this a workaround for a temporary glitch? Or is it something that must be turned on as of very recently (like this week)?

I only send via my PN home broadband with static IP. I am as far as I'm aware fully migrated to greenby and have been for some months. If that is relevant.

Re: The '451 4.3.2 Internal server error'

keeka — Wed, 30 Sep 2026 12:32:38 GMT

After posting and reading to the end of the other thread, I see relay has just moved to Greenby and they will not relay non-hosted domains. Oh well.

Re: The '451 4.3.2 Internal server error'

Townman — Wed, 30 Sep 2026 12:42:45 GMT

Time to use the email services of the party who hosts your domain name.

Re: The '451 4.3.2 Internal server error'

keeka — Wed, 30 Sep 2026 13:47:22 GMT

I knew you were going to say that.

Re: The '451 4.3.2 Internal server error'

PhilipHeyes — Wed, 30 Sep 2026 14:58:55 GMT

451 4.3.2 Internal server error - we have been seeing this error sporadically from the Greenby SMTP platform for months,
a known issue, perhaps when the platform is busy or having maintenance / cleanup. It typically clears on the next send / receive & the message goes, but it can trip up webmail resulting in a failed send & lost message.

The bit that changed overnight was relay.plus.net migrated to Greenby and adopted the stricter sender policy of the mail.enmail.co platform.

Re: The '451 4.3.2 Internal server error'

keeka — Wed, 30 Sep 2026 17:01:22 GMT

Distinguishing temporary glitch from planned permanent change is half the battle IME. Now I know its the latter, I will find alternatives. I only need mail relaying and not mx hosting.

I think they should have made a better job of informing users.

Re: The '451 4.3.2 Internal server error'

Townman — Wed, 30 Sep 2026 17:11:58 GMT

@keeka

That is an understandable expectation, but the reality is that the mature Plusnet email service had its roots in a very technically rich environment which was exploited to its full by users, including the somewhat frowned upon relaying of third party domain email, amongst other light bending activities. I suspect that such has brought about many of the difficulties in keeping the reputation of the service clean.

The new platform has closed the doors on such vagaries; the outbound servicers will only send email for known (hosted) domains and for explicitly configured email addresses on those domains.

Re: The '451 4.3.2 Internal server error'

keeka — Wed, 30 Sep 2026 19:00:45 GMT

@Townman Understood. I have been with PN long enough to know some of the history. I expect some edge cases may have been overlooked during the migration of the inbound, then outbound, components of the service. I also understand that many users aren't interested or affected by details of the implementation.

Having discovered this one's permanent, I've just gone and signed up with smtp2go. Quickly tested it and it works for domain wide from addresses. Now to understand how it achieves this from 3 new CNAMEs in my DNS. The approach is something new to me. My pre-existing spf and dkim records would now seem redundant as provider handles both.

Re: The '451 4.3.2 Internal server error'

Townman — Wed, 30 Sep 2026 21:04:17 GMT

Smtp2Go could be a sweet mitigation of the changes. Seems odd that anyone would set up a free to use SMTP service for anyone to use … for whatever purpose.

Re: The '451 4.3.2 Internal server error'

keeka — Thu, 01 Oct 2026 04:50:19 GMT

I'm sure I'll encounter the pay off. They do bulk mail but the mail servers' reputation seems ok. My email volume is low, so I fall well within the free tier. Now, to configure it so that it behaves as near as possible to a normal relay in respect to bounces. Otherwise you won't see these unless you log in to their portal.