> What happens if your mail-server goes down, for some reason?
> Are you sure incoming email will not be lost?
RFC2821, especially sections 22.214.171.124 and 4.5.4, is illuminating :)
You can expect upstream senders to keep trying for up to a week, so
that should give you time to take action in most cases. Depending on
budget/complexity, you may wish to provide a secondary MX in your DNS
(ie. a backup location to deliver to if the preferred delivery point
is unavailable) and run another mailserver there that will deliver to
the main server once it comes back up, so you know the mail is safe.
A great many ISPs will have a bot that tries periodically to connect
to port 25 on their IP range and attempt to send mail to themselves
through it - trying to weed out open proxies that can be used to spam.
In the case of my mail server, the ISP will also text me if it doesn't
answer at all, which happens about once a year. I don't think any
Irish ISPs offer such a facility, but it's not too difficult to
imagine including your mail server in whatever monitoring regime suits
Oh, and Postfix+Courier on Debian here, moving to dovecot once I get
me some of that copious free time.
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