Due to a high volume of mail abusing our services we have a rate limit in place, even if you fall under the following rate limits your account will be terminated if it's used for sending spam.
From PHP you cant send more than 20 emails per day, 10 per 2 hours, and no more than 2 in 10 minutes.
From SMTP you cant send more than 120 emails per hour, and no more than 5 in 2 minutes.
We do monitor our mail usage and will view email stuck in the que (unable to send) to see if the contents are spam. When a email is stuck in the que it's usually due to the receiving server giving an error or denying the message delivery due to a problem.