feature request: list-id mail header

[andrewcmyers]

From a user:

Some people commented to me that they use filters such as procmail to
sort the mail from different mailing lists into different folders. When
somebody sends a poll to all subscribers on a list, the poll email
doesn't go to the desired folder.

Would you consider providing an option where the List-Id header can be
set when adding a group of email addresses to a poll? All invitation
emails would then contain the header value specified by the poll admin.

Another way to please people may be adding support for the
List-Unsubscribe header value to be provided by the poll admin.

It would be most useful if these could be set to different values for
each batch of email addresses that the admin uploads, or maybe the CSV
file could have additional columns for these values.

This would be useful for anybody who uses the service in conjunction
with mailing lists managed by Mailman and similar tools.