Occasional missed sbcglobal email to Mac mail

I have an sbcglobal.net mail account in my Mac mail app. Recently noticed not all incoming messages were received in my computer's Mac mail, but were received in the on-line att.net account & on my iPhone.

As a test, I sent 5 test messages to the sbcglobal.net email account. All 5 arrived at the online mail, 4 arrived at the iPhone & only 3 made it to the computer Mac mail box.

Anyone had any problem like this with sbcglobal mail? Can't find any setting in the att.net setup that would indicate why the messages are being removed from the server before the computer gets them, but it is like they are only on the server a short time & the other 2 mail boxes get them first & they are removed before the computer gets them.

Any help narrowing this down before I try calling att again?