NMS: Not sending ALL messages

[Benl]

I hope anyone can help, since I am stuck with this issue for some time now and have not found a solution.

I am tying to send a newsletter using NMS 2.2 to approximately 1200 people.
NMS is configured to send 30 messages per batch, 500ms delay between messages and 10s delay per batch.
CMSmailer 1.73.14 is configured to send using smtp, testmessage works like a charm.

When sending the messages NMS tells me everything is fine. No errors whatsoever.

Feedback from the people who are supposed to receive the message tells me only a small percentage of them does so.

Any tips? Configuration settings/logging/message content/... ?

I appreciate the help.