Tom,
I apologize for the frustration. There were a few quirks with sendmail (SMTP) and ListMail. I have fixed a major one in the most recent distribution (v1.77b), which you might want to re-download. One problem is that, unlike other mailers, sendmail can perform it's own checking as to whether an address is deliverable. This resulted in abnormal SMTP responses and a ListMail error, resulting in failed sending. ListMail now recovers gracefully from these responses, and will soon interpret them. There is one more problem with SMTP that is being investigated and that is "random" service disconnections. If you still experience stoppages in your mailings with the latest revision you might want to try setting your "Reconnect every X emails" SMTP setting to a lower number. I can't guarantee this will help, as I do not know why such servers randomly disconnect. I am working on detecting and recovering gracefully from these types of interruptions.
Regards!