Is it POP or IMAP?
Who is your eMail provider, the part after the @ sign?
In Mail's Window Menu, choose Connection Doctor, any red dots for status, if so what is the message?
Then click the Show Details button & Check again.
The receiving email ports are:
IMAP is port 143
IMAP-SSL is port 993
POP is port 110
POP-SSL is port 995
Outgoing ports are...
SMTP and SMTP-SSL is on ports 25, 587 and 465. Port 587 has to be SSL, and port 465 is enforced TLS-wrapped and is generally used by Outlook users.
LIkely when using another ISP for your Earthlink eMail, you'll need authentication & maybe SSL.