SMTP timeout on batch email using one EmailConnectSMTP connection

[thumbtech]

We are doing a successful EmailConnectSMTP followed by multiple EmailSend's in a loop.

I understand that "sharing" the connection for multiple "sends" is desirable for speed, but I am getting a "421 4.4.2 node-sl2050.smtp.com Error: timeout exceeded" back from my SMTP host (smtp.com) after a number of records (100 give or take).

I really don't understand SMTP transactions and am wondering if I should expect to have to make multiple connections (maybe even one per message!) in order to send out batches of hundreds of e-mails. Is that just the way it sometimes works or should I be looking for a different issue causing the timeout?