When you are using a domain alias, email service is available on the domain alias domain, however there are some considerations.  For the following notes, consider the following scenario:

• domain: maindomain.com
  • domain alias: domainalias.com
  • domain alias: domainalias2.com
  • domain alias: domainalias3.com

Considerations:

1. There is only one "mail domain" or "post office" associated with a domain and its domain aliases.  As a result, any email account created will actually be aliased on all domain aliases.  This is clearer when examining an example.  If I have the above domain/domain alias configuration, and I create email user someuser@maindomain.com, it will also be aliased as:
   • someuser@domainalias.com
   • someuser@domainalias2.com
   • someuser@domainalias3.com
2. I.e., it's one mailbox, accessible by any of those names.   A mail sent to any of those addresses, will be received into the mailbox for someuser@maindomain.com. 
3. If you want to use one of alias names exclusively for mailing, you can... you just need to configure your email client in a certain manner:
   • configure the email address to use the alias name, i.e.: someuser@domainalias.com
   • configure the server login information to use the domain's name, i.e., someuser@maindomain.com
   • The mail will still go out addressed with someuser@domainalias.com as the sender/from address, and as the reply to address. The someuser@maindomain.com is used ONLY for authenticating to the mail server for sending and receiving mail... because it is the actual "mailbox". 
   • An example of configuring a mail client (Windows Live Mail, Outlook, Outlook Express, etc) is given below, simply CLOCK the image for a bigger version of it.