If you manage email with multiple domains (not alias) with Google Apps and need to recreate catchall capabilities for a non-primary domains then you can use Google’s “address alias”. I thought this was only a @gmail capability, but domains on Google Apps for Business provide this as well.
Here’s what I’m talking about.
In Google Apps you have a primary domain and ONLY for that domain can you assign a true catchall account. So for instance you’d have something like this:
admin@domain-primary.com
ed@domain-primary (this is set as the “catchall”)
However, if you want to manage a 2nd domain through Google Apps and provide them with email you cannot assign a catchall account.
admin@domain-secondary.com (can’t assign as catchall)
ed@domain-secondary.com (can’t assign as catchall)
There is such a thing as an email alias, so you could setup a number of things:
ed@domain-secondary.com
twitter@domain-secondary.com
twitter-account2@domain-secondary.com
However, with this method, your workflow is interrupted in that you can’t just shoot off aliases because you’d have to add it through the main Google Apps management console.
What you can do however, is use Google’s “address alias” to essentially do what you had previously relied on a catchall to handle:
ed+twitter@domain-secondary.com
ed+twitter2@domain-secondary.com
You can on a whim add a third:
ed+twitter3@domain-secondary.com
Now, I know there are many ways to handle emails by tagging, forward, etc.. but I had a user with particular ways and  workflow she liked so I had to do something that maintained that.
So, by using Google’s “domain alias” you can emulate a “catchall”/creating distinct emails per service without having to register them as an email Alias in the Google Apps manager.
References:
Using an address alias
https://support.google.com/mail/answer/12096?hl=en
Email aliases
https://support.google.com/a/answer/33327?hl=en
Catchall address
https://support.google.com/a/answer/33962?hl=en
Limitations for multiple domains
https://support.google.com/a/answer/182081?hl=en
Catch All and Secondary Domains
http://productforums.google.com/forum/#!topic/apps/zjgh6pEMCXk
This is a little wild, but I’ll put it here anyways:
Smart Trick to create Google Apps catch all for secondary domains
http://www.gappsfix.com/creating-catch-all-account-which-works-with-multiple-domains-in-google-apps/
