I was using PHP mail() to try and send out "activation" emails. The mail was sending, but it wasn't coming into my mailbox. My webhost put it this way...

"So far your tests are failing because what you are using [ php mail() ], is trying to
use the server itself to send out emails and that's an outdated way of
delivering emails, since those are not authenticated, thus are rejected
as modern email providers will only accept authenticated emails coming
from the email provider the domain uses."

Basically, mail() isn't a great way to send out emails. In layman's terms ... other mail servers DON'T LIKE IT. Because its not authenticated, they may flag it your email as SPAM or a potential security risk, and reject it. So, mail() will "successfully" send it ... but it just wont go through. I'm attempting to use PHPMailer myself to resolve the situation...