mail.customerdomain = works when domain is pointed and propagated
serverdomain = always works
In a scenario where...you may need to access the account or connect to the host smtp or imap or pop... the server domain will never fail.
For example I had to run a IMAP Sync from a Microsoft Exchange server the other day.
I had to use...
mail.customerdomain > FOR SOURCE server (cause it was still connected to microsoft)
serverdomainhere.com > FOR destintation (on my server)
Then the customers username/password etc.