Transport Rule with smtp relay Email (Without using authentication)

Hi
I have created a Transport rule for the  emails  from outside the organisation will tag a word external in Subject.And except if subject includes  words external
So outside  email tag with `external` If they are replying the word external will not repeat external in subject.

Inside the organisation emails like non authenticate  SMTP relay has configured for alerts. If mail is mail is receiving from such  alerts tag word is coming on subject .
Why it's happening like that. To avoid this I have to except if own domain(test.com). Why it's like that even its from inside the organisation.

Please find the attched rule screenshot for referensce.
Before-Transport-Rule.jpg
After-apply-added-domain-Transport-R.jpg