Add address search in Received headers

[rda0]

This should help to integrate the bot in certain existing mailserver environments.

We use an existing postfix, dovecot and sieve setup to "redirect" incoming external mails to @bot.server addresses. We also use mailman for mailing lists. Both cases do not "rewrite" the To header so it could be picked up by the bot. A separate postfix (configured as in #72) on bot.server handles mail for the bot, which also accepts internal mail directed to @bot.server.

This enables searching for addresses in Received headers, which is the only source of the room addresses when using redirects.