The mail-gateway parses a mail's subject line to
extract various information from it. It expects
the first tokens to encode the issue.

I'm using a roundup tracker in conjunction with
a (mailman) mailing list, where users receive
mails with the subject line having the [mailing-list]
prefix inserted by mailman.
If they do a reply, the roundup mail-gateway will
see subject lines such as [list][issue24], which
currently isn't understood.

Logged In: YES 
user_id=6405

You can configure roundup to use different brackets. 
Beyond that, attempting to guess whether there's a mailing 
list id there or not would be hard... If you can supply a 
patch then I might consider it.