Better to use the 'short name / User ID' field for this. It is a multivalue field, so you can have as many as you like without cluttering up the main notes names (as in user names).
Keep in mind: Avoid to create duplicates between INTERNET ADDRESS and SHORTNAME/USERID, then it won't be any 'fun' anymore...
Tip: Create a design protected view where all aliases (short names) and internet addresses are listed, it is the first place to check if there is reported non-received mails. Duplicates in either type will be a problem...