I remember seeing this in our DT Register, but now I'm seeing it in DT Donate. We are running DT Donate v2.2.3a in Joomla v1.5.22. Knowing that it likely had something to do with the admin notifier email I removed one of the email addresses from the two that I had there (separated w/ a semi colon) and the error went away.
So can more than one person receive the notifier email (after someone makes a donation)? If so, do I need to separate the email addresses with something other than a semi-colon? Do I need to upgrade?