How does Zendesk assign the ticket form for tickets received via email?
When a ticket is created from an email the default ticket form is set automatically upon submission. The form is set due to the default email address set for the associated brand's support address. This is helpful when you have multiple brands, and each brand utilizes a different ticket form.
If you would like to customize the automatic ticket form assignment from email submissions, this can be done with the use of triggers. Triggers will supersede default ticket form assignment for email tickets.
For more information on how you can utilize triggers and ticket forms, see Trigger resources.