"there is no direct connection to hold open." ...Huh? I guess it depends on how you define things. If your Email server is going to transfer an Email to my Email server, it opens a TCP connection to port 25 on my server and using the SMTP protocol, holds that connection open until my server either closes the connection or yours does.
In this particular case, your server starts the process (I.E. MAIL FROM:... RCPT TO: ... DATA ...) but when you send the '.' on its own line to signal the end of the message, my server does nothing like its thinking the '.' is still part of the 'DATA' segment. Since your server thinks its already sent the message, its waiting to see the '250 OK' message back from my server, but my server is still accepting input for some reason. One side or the other times out and closes the connection. My server aborts the message and never sends it on. Your server (may) think its successfully sent the message and deletes it. Big problem for me
I hope that clarifies things.