Mac Native Email Problem

I have Mac 10.4.11 and I am using its native "Mail" application for email (Version 2.1.3 - 753.1)

Is there a way to add an email address into this email program so that it becomes accepted as a "safe" or non-spam email address?

I am having trouble receiving one particular email address on a consistent basis. I receive email from this address on a non-consistent basis, sometimes yes, sometimes no.

I have attempted to "green light" this email address by placing it in the address book, but I don't know what else can be done. It is not in the spam email list accessible in Preferences - Junk Mail - Advanced. And the junk email setting for "training" is activated.

Also, I have not noticed that messages from this email address are being diverted by my server.
The sender has also confirmed that my email address is spelled correctly on their end.

Thanks for any tips on what might be causing this email address from consistently reaching my inbox. Is there an update to the native Mac Mail client from version 2.1.3 - 753.1?