Strange bug I can't reproduce
Hello,
I have an application that populates the Reply-To header and sends the e-mail to the list. Under very specific conditions, Mailman may malform this Reply-To header. What I found out is that the second address must match the pattern "Some Name" <a.b0@example.com>. The e-mail is properly formatted when it gets into Mailman, but when it is sent out to list members, it gets malformed to Some@serverhostname.example.com. Some more info: the first address name is UTF-8 encoded and then "Some Name" wraps into the other line at the middle.
I tried to reproduce it like this: Reply-To: =?utf-8?b?QUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQQo=?= < list@example.org>, "Some Name" <a.b0@example.com>
But the problem is not triggered. The problem seems to only present when I send the mail from the application, and so it would be easy to blame the app, but again when I check the queue files, it seems to send the e-mail correctly. It really seems to be a problem with Mailman, I just can't find the exact conditions to trigger it.
For privacy reasons I can not share the exact copy of the e-mails on the list, but would it be possible to send it privately so devs could find out where might be the problem?
Best regards, MegaBrutal
On 12/14/25 4:00 PM, MegaBrutal via Mailman-users wrote:
I tried to reproduce it like this: Reply-To: =?utf-8?b?QUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQQo=?= < list@example.org>, "Some Name" <a.b0@example.com>
Folding on white spase within a quoted string is allowed by RFC 5322.
For privacy reasons I can not share the exact copy of the e-mails on the list, but would it be possible to send it privately so devs could find out where might be the problem?
You can send it off list to me.
-- Mark Sapiro <mark@msapiro.net> The highway is for gamblers, San Francisco Bay Area, California better use your sense - B. Dylan
MegaBrutal via Mailman-users writes:
What I found out is that the second address must match the pattern "Some Name" <a.b0@example.com>. The e-mail is properly formatted when it gets into Mailman, but when it is sent out to list members, it gets malformed to Some@serverhostname.example.com.
This looks more like the kind of thing that an MTA sometimes does, rather than Mailman. Mailman does not modify addresses ever as far as I know, although in some cases it will substitute a different, fixed address for the original one in the message. (The three cases I know of are rewriting From to contain the list address for anonymous lists and DMARC mitigation, and it can be configured to substitute a fixed address in Reply-To.) Mailman doesn't do the kind of editing you describe, as far as I'm aware.
Is example.com your domain? If you can look at the MTA configuration for that domain, check for address rewriting in the configuration.
The easiest way to test it is to install the Mailhog application on the same host as Mailman, then configure Mailhog to send to the same SMTP gateway as Mailman, and configure Mailman to send to localhost and the Mailhog port. Mailhog will then capture Mailman's outgoing mail, you can check it in Mailhog's queue directory. Then you can release any non-test emails from Mailhog. If the messages captured by Mailhog show the problem, then it's probably Mailman.
You can also get the same effect by just shutting down the MTA, and looking at the message in Mailman's 'out' queue (or perhaps the MTA's 'in' queue for some MTAs). But that would affect all incoming and outgoing mail, not just Mailman.
-- GNU Mailman consultant (installation, migration, customization) Sirius Open Source https://www.siriusopensource.com/ Software systems consulting in Europe, North America, and Japan
participants (3)
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Mark Sapiro -
MegaBrutal -
Stephen J. Turnbull