Email address shows @outlook.com vs @bellsouth.net

I have a friend who is telling me that when replying to my email he thought they were getting to me but they were not.  after looking again and attempting to send an email he sai he noticed the return email was popping up as @outlook.com instead of @bellsouth.net. 

My email with Yahoo/ATT is an @bellsouth.net aaddress

not sure why this would happen but may be happening to others as well, and I'm not aware of an email account at @outlook.com

What is going on????

regards,

Mike