I have a set of filters that auto add emails sent within a particular domain as 'internal'

e.g. if me@example.com sends an email to boss@example.com, it is tagged internal

The problem is, my filter is not truely internal, because say I send an email to person@example2.com and CC boss@example.com, it picks the latter up and flags it internal, but this is not an internal email.

I need to filter emails sent to a particular domain, and **only** that domain.