Posting non-changeable email posts

Hi, I am using Postie to post emails from various email sources to specific categories.

For emails that I can change the subject title to [category] get assigned to the given category ok.

The emails from external sources that I don’t get the option to change the subject title don’t, however I do know what the subject title is going to be.

How do I get postie to automatically assign the known subject title to a category please?

https://wordpress.org/plugins/postie/