Hi,
We're looking at offering store and forward email service.

What we're wanting to do is when a customer is running there mail server in house, all mail goes to there server (we're hosting dns and www) if the mail server on the dsl line goes down we have our "store and forward" mail server as the backup MX record.

What mail software would allow us to do this?

All MTA can do it: Exim, Qmail, Sendmail etc. Just read up on your documentation for the MTA you are using for allowed hosts relaying. If you can tell us what you are using, more specific replies will be given.

And your mail server will need an MX entry with a lower priority set.

Here in Aust. many of the large ISP's such as bigpond will fail any mail when it can't send to the remote mail server for 5 hours (after trying every 30 mins).

What we're after is if our customers adsl line goes down over the weekend (down goes mx1) - we store the mail on mx2 so our server will receive it (ours is mx2).

It will then try and forward to mx1 (on customers adsl line) untill it get's a connection - so no email is lost. Also we want to be able to support semi-perm ISDN users that run a mail server locally too.

Business grade ADSL is quite good connection wise as most have an SLA of some type - so it's not a bad connection...

That aside is there any software that will store all mail and remember the user@ and then forward it to mx1 if it was down and it collected it.
So say if spam was sent to ceo@domain.net.au and mx1 bounces mail for accounts that don't exist and ceo don't exist mx2 (store and forward) would forward this to mx1 who would bounce the mail.
Is their anything that would do this? Looking at commercial software as well.

Basically all your need is a backup MX server that is configured with the specific domains/hosts that you want to allow to accept emails from if it is sent to it.

What controls whether it is sent to it is the MX records, and the priority setting. You would want to set up MX2 to point at your backup MX server with a priority that's higher than MX which typically is 0 (so even 1 would work)

On the backup MX, you would need to configure your MTA (whichever you use) the hosts/domains it will accept emails for, and relay it. So when the first MX goes down, MX2 will start receiving emails since the DNS will switch to MX2 when the first one fails.

MX2 accepts the emails and tries to relay it on to the first MX which is down. They will then keep trying at intervals over a few days (depending on the settings of your MTA) and when your main email server goes up, it will then successfully delivers the mail it collect and the main email server will deliver them to the various mailboxes according.

You don't have to setup user accounts at all on the backup MX server at all. I hope this is now clear enough.

Originally posted by sprintserve
Basically all your need is a backup MX server that is configured with the specific domains/hosts that you want to allow to accept emails from if it is sent to it.

What controls whether it is sent to it is the MX records, and the priority setting. You would want to set up MX2 to point at your backup MX server with a priority that's higher than MX which typically is 0 (so even 1 would work)

On the backup MX, you would need to configure your MTA (whichever you use) the hosts/domains it will accept emails for, and relay it. So when the first MX goes down, MX2 will start receiving emails since the DNS will switch to MX2 when the first one fails.

MX2 accepts the emails and tries to relay it on to the first MX which is down. They will then keep trying at intervals over a few days (depending on the settings of your MTA) and when your main email server goes up, it will then successfully delivers the mail it collect and the main email server will deliver them to the various mailboxes according.

You don't have to setup user accounts at all on the backup MX server at all. I hope this is now clear enough.

That's what I was after... Thanks a lot...

Spose I have to find out how to do it with Exim...

If you are using Exim 4, you can use ACLs to do so:

You would want to read this page: http://www.exim.org/exim-html-4.20/doc/html/spec_37.html

With specific attention on the "How to Control Relaying" section.