Due to a recent upgrade of the SSL certificate for FASmail to SHA-2, some users have reported not being able to access and/or send emails using the POP protocol with Thunderbird. The FASmail team has posted instructions on their known issues page on how to resolve this.
http://it.ubc.ca/services/email-voice-internet/ubc-faculty-staff-email-fasmail/fasmail-known-issues#ssl
If you continue to have issues after performing these steps, please contact the IT Service Desk at 604-822-2008.