Second mailserver

At the office we have a mailserver configured with ISPConfig. Now we have add a second MX record with a lower priority pointing to a server in our datacenter which is also an ISPConfig server.

I guess when we shut down our mailserver, all mail will go to the datacenter untill our server is back online.

So far so good. My questions:
- How will our second server know when the first mailserver is back online?
- How long will this second server keep all mails? Forever if necessary?
- Do I have to setup this second server with all users or can I use a catch-all user?

Thanks.

 

Thanks for your reply. This will do.

However, now I have to leave the ISPConfig interface. The same way I did before I knew about ISPConfig.