"General" is where the cosmetic changes are made. These are the fields that show up in the header of the email you send out. Note the box toward the bottom that says "This account is the default for 'mailto:' URLs." If you check the box, whenever you click on an email address in your web browser, The Bat! will start up an outgoing message with the "To:" address filled in. Theoretically…
Next, click on "Transport"
In the "Receive mail" section check to make sure the information is correct; this is also where you can change the server to IMAP if you wish. In the "Send mail" section, if you're using your domain host for SMTP, and they require authentication, click on "Authentication…"
The "Use settings of Mail Retrieval" will probably work, or you can fill them in again. The "POP before SMTP" is commonly used by regular ISPs. If you check this box, The Bat! will automatically check your mail immediately before connecting to the SMTP server; thus legitimizing you as a user. Click "OK" and you're done.
Note: If this doesn't work (even though your domain host says it requires authentication), try turning it off. That's what worked for me!