I'd like to see definite confirmation that the email goes out and gets back to you by testing both cases where the firewall is a:) enabled and b:) disabled. Can you send test message addressed to yourself out to the server and receive it in both firewall cases or one specific case. That would indicate to me if the email that your sending, or attempting to send is really getting out to the server and back again.
For what it's worth Rogers Canada employs a check on the outgoing mail to determine if it contains a Rogers return address or another user specified return address from a different server. If you don't have a Roger's return address the mail is refused and you will receive an error. There is a way to allow other return addresses however. You have to adjust your Rogers email account settings to accommodate the other return address. You do this by accessing your account online, locating the email settings page and entering the additional email address in the appropriate data entry window.
Are you in this type of situation where you send outgoing mail via Bell Sympatico, but receive email via another, non-Bell server??
Is this problem a recent problem due to a new account with Bell or new installation of the Comodo FW? Have you had success in the past with the combination of Pegasus Mail, Comodo FW and Bell Sympatico and only recently had this issue?