You need to have authentication enabled for the SMTP server.
In other words, somewhere, in the settings of Pegasus Mail, there will be a box to tick, where you can ask it to send a username and password when sending mail.
From the Pegasus Mail FAQ's:
"You may also find that your ISP requires SMTP authentication in order to send mail via their servers. Pegasus Mail supports SMTP authentication using two separate methods, called CRAM-MD5 and LOGIN. You will need to configure Pegasus Mail to use SMTP authentication using the options on the SMTP page of the Internet Options dialog on the Tools menu."
The settings you need are:
Server name: mail.yourdomain.com
Username:
you@whatever.comPassword: yourpassword
