I just added AAAA records for my mailserver on our internal network, and everything seems to work fine... except Thunderbird. Logins take 5-10 seconds over IPv6, but with IPv4 everything happens instantly. However, Firefox, which I assume shares most of the IPv6 code with Thunderbird, does not have these slowdowns when visiting IPv6 enabled sites on our LAN (hosted on the same machine that runs the mail server) Any clues to what causes this problem? Edit: Nevermind... I found the problem. I forgot to add a reverse DNS zone for my subnet, and Cyrus IMAP waited for a timeout before allowing the login to proceed.