You can set up an automation rule under Admin > Workflows > Automations > Ticket creation > New rule to add tags to a ticket when it comes from a particular domain.
In Tickets > If Requester email > Contains any of > sauls.com
Add tag > Sauls
This sample rule will check the requester's email address for the domain @sauls.com and when the condition satisfies, the tag 'Sauls' will be added to the ticket. You can do the same for required domains and create multiple rules. Remember, the order of the rules are important. Make sure to place the most important rules first in the list. For every incoming ticket, this automation will execute the first matching rule and stop.
Or you can choose the 'Execute all matching rules' option by clicking on the gear button above the rules list for executing all the rules on tickets.