Do not loose mails : MX fields and backup

If, for reasons, your server is offline for a few days or a week, mails could be lost. Normally, senders try again for a while if they couldn't deliver a message.

However, you can plan secondary mail servers to keep your messages if yours is unreachable. You only need :

It will keep messages for your server in queue and will deliver them as soon it gets back online.

On your side, add a new MX field in your DNS zone with a bigger weight. This field points to the secondary server :

@                     IN MX 10  athome.tld.
@                     IN MX 70

In the above example, your server is ligther (10) than the secondary (70). This means a sending server will try first to deliver on your server, then on the secondary if the first failed to answer.

On his side, your friend just has to add your domain as a backup. If he uses smtpd, then /etc/mail/smtpd.conf should look like this :

action relaybackup relay backup mx "athome.tld" 
match from any for domain athome.tld action relaybackup

Of course, you should return the favor 😉.