Mac Mail not sending even though I hear the 'sent' sound
If I open a New Message to compose and send to another of my email addresses I here the 'swoosh' of it sending,(and I can see it says Sending Message) but it never arrives and there's no record of it in the 'sent' box. It disappears totally.
However, if I reply to an email then the email does send and there's a copy in the sent folder.
Mail Connection Doctor gives green lights for SMTP and IMAP.
The only time it has sent a 'brand new' email (that wasn't a reply) was when Calender generated an email to remind me of an appointment. Even then it sat in the Outbox for ages and I kept having to press 'send' to get it to move.
If I send an email from signing on to my account online, then it sends the message, and the message somehow appears in the Sent folder of Mac Mail.
In short I can send emails if they're replies, but if I hit the New Message button and send one from scratch they don't arrive and disappear completely!
(I've deleted - and then reinstalled - the account on Mail twice).
Mac mini