Unable to add alias

Comcast hosts our domain email, and has for many years now.  For at least the past 4-5 years I had a 'forward' set-up so that any email sent to service@ourdomain.com was forwarded to user@ourdomain.com.  That user has since left, I deleted that forward and for over a week now I have been daily trying to add service@ourdomain.com as an alias for another of my users.  I have had absolutely no success.  I can add other addresses, but not service@ourdomain.com.  Is there an issue using the word "service" or something that I'm just missing.  On a similar thought - is there a user/admin guide anywhere for the Comcast email system, it would be very useful to have some sort of reference, especially has the business class website has change drastically over the years.

Thanks,

Bryan