Reverse DNS and multiple domains on same IP

[kdiamond]

Thank you Klug!

Things are becoming clearer now.

So all secondary domains MX points to the primary domain.
domain2.com MX(10) mail.domain1.com

I just don't understand what "anotherserver" should be on the primary domain?
domain2.com MX(20) anotherserver.domain1.com

Thank you
Br,
kd

[Klug]

For any domain you host the mails for, you should have a secondary MX.

This way, if the primay MX is down (internet access goes down, you're hosting at home) the mails that are sent to the domains will be accepted and stored on the secondary MX.
As you control the secondary MX, you can set it up to keep the mail for several days (if needed) and/or send all the stored mails to the primary MX when you want it.
This is the anotherserver.domain1.com, it should be hosted somewhere else than the primary MX.

If you don't have a secondary MX that you control, if the primary MX goes down, the emails are stored on the sender MTA.
It will try to deliver it once per hour, then once per 4 hours then once per day, most of the time for 2, maybe 4 days.
If the mails cannot be delivered (because primary MX is still down), the email will go back to sender with a Non Deliverable Receipt.

You do not control how long and how often the sender MTA tries to deliver mails.
That's why a secondary MX is nice to have.

[kdiamond]

Yes, makes perfect sense.

Thank you for the explanation!

Br,
kd

[L. Mark Stone]

I will also add that many senders attempting to delivery things like newsletters often will not retry failed deliveries at all if your mail server is down and you have no backup MX.

At least here in the US, backup MX services cost as little as $25/domain per year (more if you want more maximum storage and/or longer queue times).

Hope that helps,
Mark