When creating mail aliases for local destinations, there are currently two different ways the e-mail addresses can be specified. One is to enter the system userid, such as username.domain.com, and the other is to enter the primary e-mail address, such as username@domain.com, which in turn is then resolved to the userid upon delivery.
Although these different specifications have the same result, they can be a bit confusing to manage due to the inconsistency. The most important difference though is that aliases entered as userids show up in the "Additional email addresses" box on the associated user mailbox details page. On the other hand, aliases entered as local e-mail addresses do not show up here at all.
Ironically, if aliases are entered from the user mailbox details page in the "Additional email addresses" mailbox, here they must be specified as e-mail addresses. But then on the aliases page, they show up as userids. This adds to the confusion.
My suggestion is that the "Additional email addresses" box should recognize both types of aliases, whether they are entered here as userids, entered on the aliases page as userids (as both of these work today), or entered on the aliases page as an e-mail address. Additionally, I think that new e-mail addresses entered into the "Additional email addresses" box should show up as e-mail addresses on the aliases page instead of showing up as userids. Granted, this may be slightly less efficient for mail delivery due to the recursive lookup, but I think this is negligible and makes management easier and more consistent.