Matrixise from:

 https://github.com/python/bugs.python.org/issues/36

suggested

> Because I would like to sort my incoming emails or follow an issue,
> I need to search on the subject field. I would like to propose a new
> header for the email, just the number of the issue. Do you think it
> is possible?

My response:

This is currently configured in the code. Specifically roundupdb.py in
send_message. Search for X-Roundup. This would be good to add
upstream. I am actually surprised it's not there already.

However I suggest X-Roundup-issue-id rather than number. The field
that holds the number is id, so this makes sure that the header name
won't collide with a new "number" field.