Apple Mail is receiving emails that do not come from my mail servers. Why?

Apple Mail is receiving emails that do not come from my mail servers.

I have two email servers, @sbcglobal.net and @comcast.net, that are accessed by Apple Mail.

But I am receiving emails that are Not addressed to Me and are from Other Servers such as @cox.net, and @windstream.net.

In Apple Mail, I am also receiving emails addressed to "Undisclosed recipients" and to me, using my correct "user name" (F...9), but from @att.net.

I thought that because "@sbcglobal.net" was replaced by "@att.net" for "newer" Yahoo/ATT users that somehow this was where the @att.net emails were being sourced from. But, none of the 77 emails, that were addressed to "F...9@att.net" and listed in Apple Mail, were sourced by the @sbcglobal.net server. Interestingly, I did find four (4) emails addressed to "F...9@att.net" on the @sbcglobal.net server. But none of these appear to have been downloaded into Apple Mail.

I have tried sending Test Messages to the "F...9@att.net" address because the name matches my "user name". But the Test Messages come back as "Delivery to the following recipients failed permanently". So, if it not a valid address, how are these messages being delivered to my Apple Mail?

I thought Apple Mail only collected emails from accounts defined in "Internet Accounts".

Can anyone explain why this is happening and/or how to stop this have occurring?