Same for me. MacBook Pro OS 15.3.2.
It was intermittent at first and I suspected my email provider. However, today it is nearly every email stuck with error message about not connecting and I tested with multiple email providers - work email, iCloud, etc. - all failed. The emails that were stuck would also multiple send even though they were still in the Outbox.
I tried everything, then moved to a hotspot and things seem to work from hotspot. I don't think it is the wireless, per se (Xfinity), because if I'm on wireless I can send no problem from my iPhone. Is there something in the way it tries to handshake over wireless?