When you proceed to the signup stage you will be asked to choose an account name from one of the following categories.. This will also form your email address and, if applicable, web site address. For your email address, you can have an unlimited number of names, all with the same ending - your account name. For example, if you chose bloggs.abelgratis.com as your account name, then you could use any of the following email addresses:

joe@bloggs.abelgratis.com
fred@bloggs.abelgratis.com
sales@bloggs.abelgratis.com

In other words, you can make up the part before the @ sign at any time, and the part following the @ sign is the account name - which you will be asked to choose in the sigup procedure.. Obviously each account must have a unique name, so you must therefore choose one that isn't already taken. It is always advisable to type your account name (login name) and password in lower case characters. 

NOTE: During the signup procedure the choice of all Domain endings are given in alphabetical order, not categorised as below: