I have a cURL based API submission page that shoots emails to one of the contact lists on my account. Out of tested 5 emails only 3 went through. What could be causing this?
With the symptoms you have described, there are a couple of potential causes that are more likely than others:
One thing that you can do to help with isolating the issue is to check for the response to your CURL request though PHP and collect the HTTP status code that is returned. If there is an error when adding a contact, there will be an error message and corresponding status code sent as a response.
If you are able to get this error message and status code, we should be able to give you further assistance with getting this resolved.