We recently had an issue that I tracked down to an outdated entry in my route.cfg file.
Got the problem solved, but in the meantime, I've got around 1000 files that piled up in the SEND directory, as well as creating a bunch of files in the DEFER and DEFER\WORK folders.

All of the files in the SEND directory begin with a capital S, e.g., S06D4ABE.099

I got around the immediate problem by downing the GWIA and its MTA, renaming those three folders and creating new SEND, DEFER and DEFER/WORK folders.

Where the system was attempting to send to the faulty MX server referenced in the route.cfg file, it replaced to domain name with the IP address, in brackets, in the email.
The email have a section that starts and ends with <IaHead> and </IaHead>, respectively.

After that, it shows the obsolete IP address instead of the domain, like this:

Lots of email stuff
EHLO nameofgateway.yourdomain.com
MAIL FROM:<xxx@xxx>
*U1* RCPT TO:<xxx@xxx>

"yyy.yyy.yyy.yyy" is the obsolete IP address

Not all of the email have that backed up in the send folder have this problem, however.

So the questions are:

1) For the email that do not have the problem, what happens if I just move them to the SEND folder?
2) For the email that do have the problem, what happens if I edit the file to replace the IP address with the proper domain name, and then move those files to the SEND folder?