# Reply-to munging

Messages that flow through the global pipeline get their headers cooked, which basically means that their headers go through several mostly unrelated transformations. Some headers get added, others get changed. Some of these changes depend on mailing list settings and others depend on how the message is getting sent through the system. We’ll take things one-by-one.

    >>> from mailman.app.lifecycle import create_list
    >>> mlist = create_list('_xtest@example.com')

Reply-to munging refers to the behavior where a mailing list can be configured to change or augment an existing `Reply-To` header in a message posted to the list. Reply-to munging is fairly controversial, with arguments made either for or against munging.

The Mailman developers, and I believe the majority consensus is to do no reply-to munging, under several principles. Primarily, most reply-to munging is requested by people who do not have both a Reply and Reply All button on their mail reader. If you do not munge `Reply-To`, then these buttons will work properly, but if you munge the header, it is impossible for these buttons to work right, because both will reply to the list. This leads to unfortunate accidents where a private message is accidentally posted to the entire list.

However, Mailman gives list owners the option to do reply-To munging anyway, mostly as a way to shut up the really vocal minority who seem to insist on this mis-feature.

## Reply to list

A list can be configured to add a `Reply-To` header pointing back to the mailing list’s posting address. If there’s no `Reply-To` header in the original message, the list’s posting address simply gets inserted.

    >>> from mailman.interfaces.mailinglist import ReplyToMunging
    >>> mlist.reply_goes_to_list = ReplyToMunging.point_to_list
    >>> mlist.preferred_language = 'en'
    >>> mlist.description = ''
    >>> from mailman.testing.helpers import (specialized_message_from_string
    ...   as message_from_string)
    >>> msg = message_from_string("""\
    ... From: aperson@example.com
    ...
    ... """)

    >>> from mailman.handlers.cook_headers import process
    >>> process(mlist, msg, {})
    >>> len(msg.get_all('reply-to'))
    1
    >>> print(msg['reply-to'])
    _xtest@example.com

It’s also possible to strip any existing `Reply-To` header first, before adding the list’s posting address.

    >>> mlist.first_strip_reply_to = True
    >>> msg = message_from_string("""\
    ... From: aperson@example.com
    ... Reply-To: bperson@example.com
    ...
    ... """)
    >>> process(mlist, msg, {})
    >>> len(msg.get_all('reply-to'))
    1
    >>> print(msg['reply-to'])
    _xtest@example.com

If you don’t first strip the header, then the list’s posting address will just get appended to whatever the original version was.

    >>> mlist.first_strip_reply_to = False
    >>> msg = message_from_string("""\
    ... From: aperson@example.com
    ... Reply-To: bperson@example.com
    ...
    ... """)
    >>> process(mlist, msg, {})
    >>> len(msg.get_all('reply-to'))
    1
    >>> print(msg['reply-to'])
    bperson@example.com, _xtest@example.com

## Explicit Reply-To

The list can also be configured to have an explicit `Reply-To` header.

    >>> mlist.reply_goes_to_list = ReplyToMunging.explicit_header
    >>> mlist.reply_to_address = 'my-list@example.com'
    >>> msg = message_from_string("""\
    ... From: aperson@example.com
    ...
    ... """)
    >>> process(mlist, msg, {})
    >>> len(msg.get_all('reply-to'))
    1
    >>> print(msg['reply-to'])
    my-list@example.com

And as before, it’s possible to either strip any existing `Reply-To` header…

    >>> mlist.first_strip_reply_to = True
    >>> msg = message_from_string("""\
    ... From: aperson@example.com
    ... Reply-To: bperson@example.com
    ...
    ... """)
    >>> process(mlist, msg, {})
    >>> len(msg.get_all('reply-to'))
    1
    >>> print(msg['reply-to'])
    my-list@example.com

…or not.

    >>> mlist.first_strip_reply_to = False
    >>> msg = message_from_string("""\
    ... From: aperson@example.com
    ... Reply-To: bperson@example.com
    ...
    ... """)
    >>> process(mlist, msg, {})
    >>> len(msg.get_all('reply-to'))
    1
    >>> print(msg['reply-to'])
    my-list@example.com, bperson@example.com

## Don’t CC List Address in Reply

The default behavior of a personalized list is that a reply goes `To` the message sender and the list address is added to `Cc`.

The list can be configured so that replying to a list message replies only to the explicit `Reply-To` header and does not include the list address in `Cc`.

    >>> mlist.reply_goes_to_list = ReplyToMunging.explicit_header_only
    >>> mlist.reply_to_address = 'my-list@example.com'
    >>> mlist.first_strip_reply_to = True
    >>> msg = message_from_string("""\
    ... From: aperson@example.com
    ... Reply-To: bperson@example.com
    ... Cc: cperson@example.com
    ...
    ... """)
    >>> process(mlist, msg, {})
    >>> print(msg['reply-to'])
    my-list@example.com
    >>> print(msg['cc'])
    cperson@example.com