Listen on internal ygg address

[tionis]

Similar use case to the one described by Revertron in #8. It would be very useful to be able to listen on the internal ygg address for SMTP and IMAP services.

Another use case I envisioned would be an easy manageable backup system in which you for example use litestream for backups and when the main VM dies, a new instance of YggMail could be spun up, to which the clients can still connect with the same address.