|Image credit: Beau Giles|
To summarise with an example:
BEFORE: firstname.lastname@example.org was a Google Apps group on domain baz.com, containing the external email addresses email@example.com and firstname.lastname@example.org allowing incoming email to be forwarded to those two other email addresses
AFTER: email@example.com was a Google Apps user on domain baz.com
From an outside viewpoint, firstname.lastname@example.org remained a valid email address, apart from perhaps a very brief period when I was doing the actual work. I expected that after the work, all email would continue to be delivered. This proved to be incorrect.
Following the changes, I noticed that I was no longer receiving emails from a couple of distribution groups, which fortunately I run, using another, entirely separate, domain which is also managed on Google Apps. In other words:
email@example.com contains a number of external addresses to distribute incoming mail to, including firstname.lastname@example.org
Realising that I was no longer receiving email via this route, I logged in to the Google Apps control panel for somewhere-else.com and checked members of the group "mygroup", only to find that email@example.com was no longer present.
So where had it gone? I am the only administrator for somewhere-else.com on Google Apps, and I hadn't removed it. It seems that Google Apps very cleverly checks email addresses to see if their domains are administered by Google Apps, and creates some kind of a link if they are. That way, if an email address is removed from a domain, that removal is propagated across Google Apps to other domains even if they are entirely separate.
I can see why this is a good thing to do, as it will reduce bounces. However, if you don't know about it, it can be difficult to track down if you're performing any operations which involve the temporary deletion of an email address, such as in this case.