After transfer to new Mac Mini Outlook won't send email but no error

I upgraded from an old iMac to a new Mac Mini. The transfer process went smoothly (no errors). The new Outlook for Mac (free) did not work so I switched back to the old 365 pay version I was on previously. I receive email without issue (using POP) but emails will not send. I don't get an error as the synch appears to work fine. The sent emails just get stock in the outbox. I have checked my settings. They are the same as before and what they are supposed to be. And the old iMac works just fine (sends email no problem with the same settings).