You may need to check with your ISP or SMTP email service. Some limit the number of emails that can be sent in a single batch. You may have reached that limit with 128 or so emails.
There are many services that allow sending and tracking emails.
Madmimi and Mailchimp are two that allow many recieptiants and many emails for free.
Mailchimp has an API that can interface with Filemaker - these links detail that