User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:91.0) Gecko/20100101 Firefox/91.0

Steps to reproduce:

1. Set SMTP server name to an IPv6 address. Validation is fine; dialog accepts it. (Dialog does not accept bogus IPv6 addresses, e.g. with a z on the end, so it's definitely validating.)
2. Try to send mail.

Actual results:

Thunderbird 78.x: Immediate dialog box showing "Sending of the message failed" with no further details.
Thunderbird 91.0.3: Nothing happens. You wonder if you even clicked the Send button at all.

Expected results:

Sending mail is actually attempted instead of immediately giving up for no apparent reason.
This is the same as described in the following comment, but that bug report is also about getting these interfaces with accept an IPv6 address (validation), which is not a problem anymore: bug 1603550 comment 6

Quick correction: bug 1603550 was not about validating IPv6 addresses at all, but about IPv6 not working for SMTP at all (or at least for an IPv6-only server with a DNS name), from my understanding. Now, it works fine with a DNS name, even if that name resolves solely to an IPv6 address. It does not work with an IPv6 address directly (behavior is as I described). This is why I consider the issues not to be identical, although perhaps bug 1603550 could be considered incompletely fixed.

To further illustrate the problem, one workaround is to edit your hosts file to create a DNS name for the IPv6 address; this will have no problem sending. Just can't send when SMTP server is set to an IPv6 address directly. That was apparently not the case when #1603550 was opened, so the situation has changed.

Please use bug nnnn format, so bugzilla will provide the links

I see two problems when using ipv6

1. have to wrap ipv6 addr in [] when passing to newURI, e.g. Services.io.newURI('smtp://[::1]:25'), this is easy to fix.
2. TCPSocket doesn't seem to work with ipv6. I have a postfix on local, with the following code, socket is not opened, postfix logs show no connection attempt as well.

   s = new TCPSocket("::1", 25)
   s.onopen = () => console.log('on open')
   s.onerror = e => console.log('on error', e)
   

   This is difficult to fix.

IMAP is not affected by this, if that helps to know. I am not sure what SMTP does differently in creating a TCP connection (haven't looked at source at all).

I guess we should fix the SMTP code here, and then file a bug about TCPSocket IPV6 support separately.

Depends on D128843.

Outstanding work getting this and the m-c bug fixed. Thanks Ping!!

Pushed by mkmelin@iki.fi:
https://hg.mozilla.org/comm-central/rev/f6b911e8f862
Support using IPv6 address as SMTP server name. r=mkmelin

Comment on attachment 9246587 [details]
Bug 1728773 - Support using IPv6 address as SMTP server name. r=mkmelin

[Approval Request Comment]
Regression caused by (bug #): bug 1661694
User impact if declined: Cannot use IPv6 address as SMTP server name
Testing completed (on c-c, etc.): beta
Risk to taking this patch (and alternatives if risky): low

Comment on attachment 9246587 [details]
Bug 1728773 - Support using IPv6 address as SMTP server name. r=mkmelin

[Triage Comment]
Approved for esr91

Comment on attachment 9246587 [details]
Bug 1728773 - Support using IPv6 address as SMTP server name. r=mkmelin

[Approval Request Comment]
Resetting to approval? -- Depends on bug 1736508, that won't be available until 91.4.0.

Comment on attachment 9246587 [details]
Bug 1728773 - Support using IPv6 address as SMTP server name. r=mkmelin

[Triage Comment]
Approved for ers91

Thunderbird 91.4.0:
https://hg.mozilla.org/releases/comm-esr91/rev/84f105466886