Well, for the reference of any future Mac Mail users - what finally worked was:
POP
Incoming mail server: mail.domainname.ext
User name: name+domainname.ext
Password
Port set to 110
Outgoing (SMTP): mail.domainname.ext
Port 26 (apparently 25 was blocked, preventing me from sending)
Authentication: Password
Username: complete email addy (in my case, name@domainname.ext)
Password
I had to try a lot of things before it worked, so don't despair if you're reading this and pulling out your hair! The moderators here are really great, and referring to my cpanel helped, too. Good Luck, and thank you Moderators!