What's the TT error code? TT501 or TT502 or another?
Your mailbox has been rate limited for exceeding the number of recipients you've sent to either in any hour or any day, depending on the error code. Bear in mind that if you send to 30 recipients and then have to repeat that send very soon after then that could trigger the spam protection to apply rate limiting. It's only temporary and in the first case the full sending allowance will be restored in an hour and in the second the full allowance will be restored in 24 hours.
