Unable to resolve email sending failures via Mail app on macOS sequoia 15.5

I have an issue sending e-mail.

Whenever I press 'Send', I get a whoosh sound, but then immediately the message below appears (I've redacted the two places where our e-mail address, which starts with a 't', is mentioned).

I've tried all the buttons possible to get off this screen, but I can't; it appears every time I try to send an e-mail. The Connection Doctor reports a fully-functioning connection. I've tried sending from other e-mail addresses but the same thing happens.

I'm using a 2024 M4 iMac with Sequoia 15.5 and Apple Mail version 16.0.

Any help or advice would be very gratefully received!

[Edited by Moderator]

Original Title: Can't send e-mail