Join Complete Mail has NOT been sent.

Hi,

When a member tries signing up for a site with confirmation Emails enabled, we get this error.  Any ideas? Thanks!

Join complete
Mail has NOT been sent.
Unfortunately we could not send the confirmation e-mail to you at this time. Please, try later. We apologize for any inconvenience. Please report this bug to the administrator.

Is the email actually being sent in spite of receiving that error?

If not, is your site able to send out other notification emails such as new message alerts and such?