TT502 means your mailbox has exceeded the daily limit for sending mail messages. A temporary stop on sending has been triggered by the spam protection process. Normally you'd be able to send to hundreds of recipients each day. The full sending allowance will be returned in the 24 hours after the stop on sending was applied. If you wait 24 hours then your mailbox will be back to normal.
There are circumstances that can trigger a lower sending limit. Using mail via a non-UK network, a network where the IP address is a non-UK address or using a VPN where the VPN host is outside the UK.
Something about the way or rate your mailbox is sending messages has caused the spam protection process to kick into action.
