Since the petition sends 9 email per signer, if 21 people sign in an hour, the hosting service shuts down outbound emails to prevent spammers from hijacking accounts and sending junk. It happens only occasionally. The fix is for me to route emails to another email provider such as Send Grid. I just don't have the time now to do the modifications. In the meantime, I run a separate script during slow periods sending any petitions that didn't go during those peak periods.