Sending all msgs, but some appear unsent (XP x64)

OE is configured to Send Later. When a batch of emails is in the Outbox, and
I Send them all, the progress meter counts through them all. All messages get
sent. However, OE will only show that 5 or 10 of them went out, and those
appear in Sent folder.

The remaining emails sit in the Outbox. When I Send, another 5 or 10 or 15
(usually a multiple of 5; not 3,4, or 17, etc.) move to Sent, and if I repeat
this they all eventually get moved to Sent. But each time I am clicking
Send/Receive, EVERYTHING in the Outbox is really getting sent, so some emails
are being sent a bunch of times. OE just doesn't think it's sending all of
them, but it is.

What I have tried:
I have compacted the folders-- no change.
I have deleted the entire Store folder, and let OE re-create everything-- no
change. Permissions look good on the folders.

What I am running:
We have OE 6, with Windows XP Pro x64.
We use Mcafee SonicWall Enforced Client, but I don't think it is the culprit.