I'm having a similar problem with my mailing....
For smaller group e-mails, I have sent a few that worked just fine, but I just tried to send one to all the families at my school (about 540 e-mails) and it wouldn't send all of them. It started and sent almost 200 messages, but then just stopped. I tried to "send all" again, and nothing happened, so I sent one or two individually, and they went off just fine. So I tried again to send all and fmSpark did send a another larger group (maybe another 50 messages) but stopped again. So each time it stopped, I had to send one individually, then click "send all" until it finally worked its way through the whole group.
Do I have a setting wrong? I was using a sample stationary that came with fmSpark. Everything else seemed to be fine... it merged with out a problem, I could see the preview before sending the mailing, I could edit the recipient list, etc. Help!