Question : How-to configure Sendmail as a secondary Mx (store and forward) ?

Hi,
I have 3 mail servers (One Microsoft Exchange and two Mailmax). The mailmax servers have 100+ domains on each. I also have a Spam filtering server (Vircom) that is used for 80% of the domains we host.
I want to configure Sendmail as a backup mail server that will receive and store mail whenever one of our server is down and will resend mail to the mail server when it come back up. I don't wan't to list each mailbox on Sendmail (this woul'd be a nightmare to manage), only the domains.

DNS entries look like this :

sendmail.hosting.com.      IN     A        100.200.200.10      (This would be the new Sendmail server)

exchange.domain1.com.   IN     A        100.200.200.1
domain1.com.                  IN     MX      10 exchange.domain1.com.
domain1.com.                  IN     MX      20 sendmail.hosting.com.

mailmax.domain2.com.    IN     A        100.200.200.2
domain2.com.                 IN     MX      10 mailmax.domain2.com.
domain2.com.                 IN     MX      20 sendmail.hosting.com.

vircom.domain3.com.       IN     A        100.200.200.3
domain3.com.                 IN     MX      10 vircom.domain3.com.
domain3.com.                 IN     MX      20 sendmail.hosting.com.

The Sendmail version installed is 8.14.3.

I found on this site many way to do it but I am not sure wich one is the best for our needs.

Thanks !

Answer : How-to configure Sendmail as a secondary Mx (store and forward) ?

In your case you should be able to use access or relay-domains, it should work either way. relay-domains is probably the most straightforward approach. "access" gives you way more possibilties (custom error messages, relaying based on single addresses etc), but you shouldn't need this. If using access you will usually have to create your hashmap after modifying the file, thus involving more steps.

I recommend to read the exhaustive on sendmail about those files, this will give you the most accurate answers.