Email alias are user-friendly addresses that Celoxis lets you use instead of the long system generated addresses.
For example, the email address generated for creating custom app item; lets say your Bug Tracking application via indirect email address looks like: celoxis+NI-17f-2433@celoxis.com. This address is complex and not easy to remember. For this purpose, Celoxis lets you use an alias which is a simple address which maps to the complex address.
Read this, to see how email filing works using email aliases.
The alias name should be different than the one you have created for your Incoming or Outgoing email address.