i have my site set to 'user activation' - user registers, site sends email, user clicks activation link, user account activated...

well everything works great except, when I finalize the registration... i get the following error message:

"You are now registered. However, we were unable to send the activation mail to your email account due to an internal error that had occurred on our server. We are sorry for the inconvenience, please send the webmaster an email notifying him/her of the situation. "

actually this is bogus because the email does go out, and when i click the activation link everything works great...

which file contains this error message, so that I may change the wording of it to suit my needs?

NOTE: when i use the Email Users featuer as admin that gives me an error message too, but the emails all go out as well.