>> Can't we find the bug number of an email by matching the To and Cc
>> addresses against xxxxx@debbugs.gnu.org where xxxxx is a bug number?
>
> No because the initial email would be sent to bug-guix@gnu.org or
> guix-patches@gnu.org, not xxxxx@debbugs.gnu.org.

Yeah, right, I forgot about the first email.

> There are also some control messages which are not sent to
> xxxxx@debbugs.gnu.org but are received by the Debbugs service through
> other means.
>
> I’m now getting the bug number by convention: mumi gets them from
> Debbugs, stores them in a predictable fashion (<bugnum> dash <msgnum>),
> and mumimu gets the bug number from that file name.

Ok, that makes sense.