1. Hover over the Admin button in Autotask and select the User-Defined Fields option
2. In the Accounts tab, click +New and create a new UDF named "Email2Ticket Domains". Your description can be "Alternate Domains (<@domain1>.com, <@domain2>.com, <@domain3>.com, etc.). Sort order should be 999, Field Type = Text (Single Line). The rest of the fields can be left with defaults. Save and Close.
3. Search/Open the account you wish to configure the UDF for.
4. Click the Edit button and go to the UDF tab. You should see the "Email2Ticket Domains" UDF listed.

When adding domains in to the UDF in Accounts, they are "@domain1.com, @domain2.com" etc. They must contain the "@" and are separated by commas. Once you have entered all the domains you wish to associate with that account, click the Save and Close button.

The default rules in Email2Ticket are configured to derive contacts from the email addresses of senders, so if the domain is matched against a UDF, it will create the ticket under that account accordingly.