As IDW says, I've been going nuts trying to get Pegasus to work with AT&T. They're changing over to YAHOO servers and seems to be causing some issues. Brian Fluet has been trying (heroically!!!) to get me running again. Made these changes in Pegasus' settings and that seems to have done it. At least for AT&T. But you might consider checking your IP if it's different:

 Incoming Mail (POP) Server

    Server - pop.mail.yahoo.com
    Port - 995
    Requires SSL - Yes

Outgoing Mail (SMTP) Server

    Server - smtp.mail.yahoo.com
    Port - 465 or 587
    Requires SSL - Yes
    Requires TLS - Yes (if available)
    Requires authentication - Yes [log into SMTP server using POP3 username and  password - pop.mail.yahoo.com]

Your login info

    Email address - Your full email address (name@domain.com.)
    Password - Your account's password.
    Requires authentication - Yes
 

When sending mail error message pops up:

 STMP network or protocol error sending mail

Protocol error- the server has unexpectedly refused or failed a request made by Pegasus mail

 When downloading mail:

 Pop 3 network or protocol error

The hose claims that either the uster name or password you have supplied is not valid.

I've not changed any user name or password. In fact I have never had a password for Pegasus Mail

Please take a look at the other currently active thread about similar issues to understand what might be going on: Just replace AT&T with your own email service provider.