automatically pass reply-to to mailgun

[guruofgentoo]

Mailgun docs require passing the reply-to value as a header. keg-mail does not currently handle this, so if message has a reply-to set, this gets dropped on send via the Mailgun API.

Current workaround is to set mailgun_opts on send, i.e.:

        flask.current_app.mail.send(
            msg,
            mailgun_opts={
                'h:Reply-To': msg.reply_to,
            },
        )

[guruofgentoo]

Fixed in 0.3.0