Emails not being delivered

Hello friends,

I have a problem sending out emails from a particular email address on my organization.
The email usually contains just some HTML text without any links or images, just a bit of bold styling. I am using a php class called phpMailer to send out this email. The email address itself is “erefs@mydomain.com”.
The problem is that some of the recepients are not getting the email at all (not even in spam).

HOWEVER, when the exact same email with the same content is sent to the recepient which couldnt get the “erefs” one from a user’s outlook account (email address “user.name@mydomain.com”) it arrives successfully.

I know there could be multiple causes to this problem, but I am just looking for what these causes could be so I can rule each one out untill the answer is found.

All suggestions would be greatly appreciated.

Regards,
Alex

Most obvious cause, an email server somewhere is blocking the emails.

Check the nobody@mydomain.com address (or forward it somewhere)

If you receive failed to send emails with error code 550, that’s what you would be looking at there.