User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_2; en-us) AppleWebKit/531.21.8 (KHTML, like Gecko) Version/4.0.4 Safari/531.21.10 Build Identifier: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.1.8) Gecko/20100227 Thunderbird/3.0.3 Thunderbird does not seem to fall back from IPv6 to IPv4 for delivering mail. With limited/broken IPv6 connectivity telnet yields the following, while Thunderbird comes up with "The message could not be sent using SMTP server mail.foldr.org for an unknown reason." [14:38:52]> telnet mail.foldr.org 587 Trying 2001:470:1f0b:15ba::1... telnet: connect to address 2001:470:1f0b:15ba::1: Host is down Trying 88.198.102.118... Connected to mail.foldr.org. Escape character is '^]'. 220 mail.foldr.org ESMTP Sendmail; Thu, 11 Mar 2010 14:39:03 +0100; (No UCE/UBE) logging access from: 1x-193-157-197-X.uio.no(OK)-1x-193-157-197-X.uio.no [193.157.197.X] quit 221 2.0.0 mail.foldr.org closing connection (Surprisingly, IMAP seems to do the right thing!) Reproducible: Always Steps to Reproduce: 1. "Break" IPv6 connectivty 2. Try to send mail Actual Results: Thunderbird does not deliver email. Expected Results: Firstly, Thunderbird should give a more exact error message. Secondly, it should probably try to fallback to IPv4.

I can confirm that this bug still exists in version 12.0.1 (Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:12.0) Gecko/20120428 Thunderbird/12.0.1). My experience is identical to Volker Stolz'. I wonder if this is even an IPv6-specific bug, or if an SMTP server with two IPv4 addresses would exhibit the same bug if the first address is unreachable. (In which case it's a recurrence of bug 86917 perhaps?) Other old IPv6 bugs that look related: bug 108108, bug 136898.

please see Bug 795113 which deals with two (2) aspects of IPv6 mailserver account problems for SM/TB. these apsects are addressed separately in the two (2) bugs, infra. Bug 199391 seems to deal with the basic code problem in that IPv6 will not work for server accounts: sending via SMTP on (usually) 465 and collecting via IMAP on 143/993 -- i am familiar with both. it also claims that POP3 is equally affected, but i do not deal with this protocol. Bug 551675 seems to indicate that the Bug 199391 problem (now going on 10 years old) does not offer an IPv4 fall-back after failing on the mozilla-preferred IPv6. one would conclude from this indication that DNS of the account's server name has an IPv4 MX in addition to the IPv6 (is multi-homed). this can be verified as demonstrated in Bug 795113. hopefully, these bugs can be consolidated into Bug 795113 and someone will finally resolve all of the issues. thank you,

Removing myslef on all the bugs I'm cced on. Please NI me if you need something on MailNews Core bugs from me.