Question : Howto create a backup mail server

I've been looking at various docs regarding how to create a backup mail server using Sendmail under Linux.  

One set of docs seems to suggest that all I need to do is to add the domain for what I want to act as a secondary mail server for to /etc/mail/relay-domains and set the MX records so that the primary mail server has a higher priority (lower number) MX record than the secondary mail server.

Eg:

adomain.com.         86400   IN      MX      10 mail1.adomain.com.
adomain.com.         86400   IN      MX      20 mail2.adomain.com.

I tried that, and it works while mail1 is up, but when it's down I get:

553 5.3.5 mail2.adomain.com. config error: mail loops back to me (MX problem?)
554 5.3.5 Local configuration error

Other docs suggest adding mail1.adomain.com as a delivery agent
or adding an alias
*@adomain.com    %[email protected]

1) Why doesn't simply adding the domain to /etc/mail/relay-domains work (assuming the appropriate MX records are in place)?

2) What's the "right" approach?  Adding mail1.adomain.com as a delivery agent feels wrong as does adding the alias.  Why should that help???  [Or do they mean
     *@adomain.com    %[email protected]
I could see why that would work, but it feels like a cludge]

I've also looked for useful sendmail docs relating to this, but with little success. Pointers please?

Answer : Howto create a backup mail server

Yep, mailertable is a good mechanism for your requirements.

While it might be "understandable" on a first reading the O'Rielly Sendmail book is the definitive reference.