Strange characters before mail address aftere validation at the receivers end.

Hi All,

One user has a problem sending mail to one peticilar company. He has no problem sending mail to anybody else.
After troubleshooting i am convinced the issue lies at the receivers end, this is why:

-Our user can send to all other but not this peticilar company
-The IT guy at the other side says the following,

"When I talked to our provider he told me, that when "user" send us an email, his name ist not "user@company.com", itis "btv1==692b47e10c6==user@Company.com" and when we do a reverse look up to your mailserver this receipent ist unkown"

So, where is that strange number with characters coming from?

Hope somebody can put me in the right direction.

thanks,

Ron