macOS Mail and OS X Mail can show you all an email's header lines—which contain possibly...How to Filter Mail Using a Custom Header in Mac OS X Mail
You can teach OS X Mail use any header line for email filtering. The Rule of Sender and Subject...Send a Message From a Different Account in Mac OS X Mail
If you have more than one account (or more than one address per account) in Mac OS X Mail you can...How to Block a Sender in Mac OS X Mail
Blocking a sender is easy in OS X Mail, and particularly so if you have a message from them at...