I use OS X Mail program on a mid 2015 MacBook pro running Big Sur. Before that I had a late 2013 Retina model running Catalina. Both of these machines have/had this problem and with some previous OS’s.
When I’m at home everything works fine. If I’m somewhere else, and they have the same provider as I do (Spectrum), everything is fine. If I’m somewhere that has a different provider, I can still receive mail as usual, but it refuses to send and stashes the mail in the outbox. When I open Mail the next time I’m at home, it all goes. To send mail using any other provider, I have to use a browser and log in as me to Spectrum’s website. That’s a pain.
I also have a 2009 Macbook Pro running El Capitan that I was given recently. The setup for the Mail program is noticeably different from newer os’s, and it does not have the problem. Send and receive both work wherever I am. I’d use it for my on-the-road mail machine, but it has other problems.
good ideas much appreciated