Email not delivered

I am trying to send a email to one of colleague but the email keep returning.
I get the message;

THIS IS AN AUTOMATIC GENERATED DELIVERY STATUS NOTIFICATION.

DELIVERY TO THE FOLLOWING RECIPIENT FAILED.

This is an email address that I normally sent email to. I spoke with my friend and he indicated that nothing has changed in his email.

Please note that this a citigroup.com email. I tried other citigroup.com email but they failed. I am sending this from msn.com. However, when I sent the email to my yahoo account and resend from there it goes through. I would truly appreciate your help because msn.com is my primary email. Thanks.

 

MSN.com is also MY Primary account and I've been having problems with it for 3 days now.
Mail is not being delivered on time, in duplicate or not at all.
It's MSN......as usual.
You can try contacting them, but when I did I didn't get any answers.

Toni