I am fairly certain neomutt will work the same
Most software firewalls only blocks incoming traffic - so unless you have setup rules for outgoing (targeting the destination port) as well - this is most likely not the case - but nonetheless I think you are looking at a network configuration error. Even though you initially strikes me as a proficient Linux user - here is a link to some basic troubleshooting.
Is the server a pop3 only server?
EDIT 2021-05-24T07:42:00Z
For the sake of testing your issue - I installed neomutt - and setup a config like yours - of course adjusted for my use - it worked OOB.
So either you are having a connection issue or as @mithrial suggests - in below comment - maybe the certificate is the issue.