Cannot send mail - the server rejected one of the recipients addresses. The message has been placed in your outbox

I’ve searched everywhere on the internet about this issue and I can’t find a solution. I get this message when trying to mail my partner - but for no one else. All outgoing and incoming mail are fine except for his email! At first I was receiving messages that his mailbox was full and he said people have experienced issues emailing him so he went ahead and deleted a load of old emails… but now my emails won’t even send to him and get placed in my outbox. Apparently it’s a ‘me problem’ but I’ve checked my settings - standard iCloud eMail acc so no smtp settings to check. The fact that ALL other emails get sent except to him is so confusing. I’m on iOS 17.4.1 which is most up to date as of today, I’ve also reset my network settings and phone etc and doesn’t help. I have tried emailing from an external mail address on my Mail app and still goes to my outbox - the smtp settings here are correct. Could it instead be a ‘him problem’? He said because it’s now not even leaving my outbox and then bouncing back to me makes it sounds like a problem my end.