Configuring Mail.app "From" drop-down menu?
There are few ways to do this.

The From field could be preselected in the mail preferences, using the dropdown and select From that would be ALWAYS used from now on.

I still have it as choice since i use multiple emails and want to choose the From.



Next is the multitude of From's, way more then actual email accounts.

Called the smtp servers list

That can be resolved by DELETING all duplicates and others, and leaving only the one you actually have and use.



In drop down Click on edit smtp list and delete the one you do not want.