Cannot send any Plusnet emails

[#1]

Good afternoon. I am unable to send or forward e mails from my account - <redacted> - or my elderly MiL's account - <redacted>.

I get the same error on requests to send new or forward existing mails - SMTP Error(550). Failed to set sender. (5.7.1 Unknown sender).

I'd be grateful if someone can help resolve these issues on both accounts.

Thank you.

Moderator's note:
Personal information removed from a public forum (to an area that staff can see).

[#2]

Good afternoon @safcdl16.

I've just picked up your other post around unarchiving now, and sent you over a private message.

Peter