topic Re: SMTP Error 451 4.3.0 Error: queue file write error. in Email

SMTP Error 451 4.3.0 Error: queue file write error.

mavison — Thu, 15 Jan 2026 12:22:22 GMT

Since migration my email client often receives a pair of errors when trying to send an email to the Force9 SMTP server:
451 4.3.0 Error: queue file write error
421 4.3.0 Mail system error

The email is not sent, but usually it succeeds when I notice and try sending it again.

This was reported to Greenby on 15th December, and since then they have kept reporting that it is being monitored and "the engineering team is continuing work on a permanent fix to make sure this doesn’t happen again"

However, it is still affecting me, and over the last 7 days it is about a 7% failure rate - and this is on an average of only 20 emails sent a day (often to other plusnet addresses!).

I have also realised that the same error is the cause of missing emails that my NAS sends me, as I have just found it has logged about 20 of these errors since migration in the last 4 weeeks. And it does not retry, so the emails are lost.

I have posted here to see if anyone else is experiencing these errors?

Re: SMTP Error 451 4.3.0 Error: queue file write error.

David79 — Mon, 23 Feb 2026 23:30:33 GMT

Exactly the same thing happens to me, after the migration to Greenby. I thought Greenby was meant to be state of the art, but I've had to contact them on a number of occasions, one of which lasted a few hours. It started with AI, but after some time I was handed over to an agent, who must have been dealing with several queries simultaneously, judging by the length of time taken to respond. At least the problem was sorted after that time.

It's very odd that on the second attempt the email is sent promptly.

Re: SMTP Error 451 4.3.0 Error: queue file write error.

mavison — Tue, 24 Feb 2026 09:55:02 GMT

@David79 I am sorry to hear you are suffering from this problem as well, but thanks for reporting it here. I was beginning to think it was just me!

I do have an open ticket with Greenby, where they claim that they are aware of the problem and are working to fix it ... but that has been the case for over 2 months now.

However, it is a rare day that I do not have at least one of these errors - yesterday there were four!

My ticket was raised in December, and I update it every two weeks or so with my latest batch of errors - please can you keep Greenby updated as well?

My experience is that normally a re-send after a short delay will work fine.