Emails not sending

I have an issue whereby my emails aren’t sending. I compose the email, hit send, it closes and goes into the outbox but then after a few seconds opens back up again. It then closes once more and opens back up again.

It’s not giving any indication that it’s an smtp issue ie. It’s not asking for a password or such like.

I’ve had the email account on the iMac for ages and it’s only just started to happen. There’s been no change in email settings. It happens with all email accounts.

I’ve tried rebuilding the email account as well as deleting the the Apple mail Prefs file, starting in safe mode, but still the issue exists.

Any ideas or help would be greatly appreciated.