Possible causes for a response triggering a new challenge?

I'm setting up Postfix+mysql virtual users, with maildrop as the MDA.
It's been quite a task and I'm close to finishing it, but alas, I get
new challenges after sending in a response.

I don't have the email address or user part in /etc/aliases, as the
FAQ mentions about Sendmail.

Does anyone know of any other common reasons for this, or what I
should look at?  Below is .tmda/logs/incoming for one message and a
response

== Challenge

Date: Tue, 12 Apr 2011 15:25:52 -0400
From: Darek M <[email protected]>
  To: [email protected]
Subj: Testing again
Actn: CONFIRM action_incoming                                           (1257)

Date: Tue, 12 Apr 2011 15:25:52 -0400
From: Darek M <[email protected]>
  To: [email protected]
Subj: Testing again
Actn: CONFIRM pending 1302636352.14229                                  (1257)

== Response

Date: Tue, 12 Apr 2011 15:33:10 -0400
From: Darek M <[email protected]>
  To: [email protected]
Subj: Re: Please confirm your message
Actn: CONFIRM action_incoming                                           (2271)

Date: Tue, 12 Apr 2011 15:33:10 -0400
From: Darek M <[email protected]>
  To: [email protected]
Subj: Re: Please confirm your message
Actn: CONFIRM pending 1302636789.14271                                  (2271)

-- 
Darek

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