email address changes from what is entered

[Ten Words]

When sending an email to a person in my addressbook, the address is changed
to the following extension: @mx03.mfg.onr.siteprotect.com. I don't know how
this is being changed.

I have checked my antivirus program (Microsoft Security Essentials) and
there is no evidence that it made the change.

Does anyone know what is causing this and how to stop it.
It keeps him from getting the emails.
Thanks.

 

[Roady [MVP]]

Does this still happen when you create a new contact with for instance the
address (e-mail address removed) (yeah, for testing purposes indeed ;-).
When exactly does this change occur?