[exim] mail to myself is rejected

If I send a message to myself using a 3rd party SMTP server, exim rejects
the mail and I receive the following message in the returned mail:

<chris@???>: host mail.northfolk.ca[220.234.29.148] said: 500 error
    (in reply to MAIL FROM command)

There is no record of the mail in either mainlog, rejectlog, or paniclog.

I can send to myself if I use my own server for SMTP.

How can I debug this problem?

--
Good day, eh.
Chris