topic Re: relay.plus.net "Unknown sender" error in Email

relay.plus.net "Unknown sender" error

stinging6148 — Sun, 14 Sep 2025 18:35:50 GMT

At some point today, relay.plus.net has started rejecting all my emails with "550 5.7.1 Unknown sender"

What does this even mean?

I thought maybe they started requiring authentication even when at home, so I turned that on, but to no avail.

This is using postfix btw with "relayhost = relay.plus.net"

I've not seen this error until about 3pm today, it started as intermittent, but has been constant since 4pm

jab1 — Sun, 14 Sep 2025 18:40:21 GMT

stinging6148 — Sun, 14 Sep 2025 18:55:58 GMT

Ah, cheers, I had not stumbled on that thread.

Seems related.

I host my own smtp server on a fixed ip.

Client does not seem to matter, but tested with roundcube, k9 and outlook when trying to diagnose.

I tried pointing roundcube directly to relay.plus.net to try and rule out postfix and get the same issue.

What freaks me out slightly is that if I connect to relay.plus.net from cli :

"openssl s_client -starttls smtp -4 -connect relay.plus.net:587 -crlf -ign_eof"

and authenticate and send an email it has worked 100% of time.

Which is why I went down the road of trying to eliminate postfix.

But I guess changes are afoot at plusnet, so will give up for today and look again tomorrow

jab1 — Sun, 14 Sep 2025 18:59:45 GMT

OK - but a tip - try sending in the first 10-15 minutes of the hour.

mwwagain — Sun, 14 Sep 2025 22:34:34 GMT

Added my issue to the referred thread as we assume it is part of the same problem.

stinging6148 — Wed, 17 Sep 2025 18:45:05 GMT

This has not occurred again since late Sunday, emails have gone through the the relay ok since then.

Silverwhistle — Wed, 05 Nov 2025 20:14:09 GMT

Hi, this is happening to me this evening, too:

Error code: -17099 Unknown sender

when I try to send mail.