Here's the reply I received from Mail.it support:
the problem you have faced is a typical limit for the maximum amount of messages that can be sent within a single SMTP session, which is commonly used to prevent SPAM by the Server configuration. As far as SMTP protocol does not allow the Client to obtain this information from the Server prior to sending the messages, this kind of Server configuration can be only recognized based on the test results.
In a 'Simple Mass Mail' example solution, that is shipped with a plug-in, we suggest to re-establish SMTP connection each time when a certain amount of messages is sent (50 messages by default). We can suggest you to use the same kind of behavior, and re-establish connection after each 10 messages.
A detailed plug-in information you may find at the mail.it 3 Manual, which you may find at the download package, available by the links below:
http://www.dacons.net/download/mailit_m ... 17.101.zip
http://www.dacons.net/download/mailit_w ... 17.101.zip
Does fmSpark provide access to Mail.it so I can change this default behavior?
Thanks.