It sounds like maybe K-9 mail has an issue where it'll open lots of
connections, which was leaving Thunderbird spewing a lot of "couldn't
authenticate" errors. Hopefully this'll fix that.
# Maximum number of IMAP connections allowed for a user from each IP address.
# NOTE: The username is compared case-sensitively.
#mail_max_userip_connections = 10
+ mail_max_userip_connections = 50
# Space separated list of plugins to load (default is global mail_plugins).
#mail_plugins = $mail_plugins