create a new field that matches the domain and search against it ...
form the top of my head:
try this anywhere to see
| makeresults count=1
| eval emails = "email@example.com;;;firstname.lastname@example.org;;;email@example.com;;;firstname.lastname@example.org;;;email@example.com;;;firstname.lastname@example.org;;;z@email@example.com"
| makemv delim=";;;" emails
| mvexpand emails
| rex field=emails "\@(?<domain>[^\.]+)"
| search domain=gmail
you might need a better regex for more complex / emails that have the
@ in the address itself