This version of outlook has some unusual, and intermittent errors.
First, while sending emails, it would occasionally generate error 0x8004210B
That was easily fixed by just restarting the PC.
But then outlook would generate a more consistent 0x800ccc78 error.
I’d get this error regardless of the email size, and regardless of attachments.
The wisdom on the net said to check that the email address is correct, check your ISP etc.
I checked all those, but found nothing wrong (this account had been working fine for months previously).
I’m starting to suspect that the ISP has made some change.
The only thing that seems to make sense, is the outlook account options (the windows that has all the server settings … eg POP3 and SMTP settings).
I click on the “more settings” button.
Then under the “outgoing server” tab, I tick: SMTP requires authentication… and use same settings as my incoming mail server.
After that, sending emails caused no more problems.