TT502 means that an excessive number of mail messages have been sent from your mailbox in the last 24 hours or that your sending has been rate limited for a security reason and this lower rate limit has been exceeded. It may take up to 24 hours for the full sending allowance to be reinstated.
Have you recently received a TT501 error or any other error message? Do you have multiple devices connecting to your mailbox? Are any devices connecting via a VPN either as part of a home security / Internet security package or a work VPN service?
The reason for asking about multiple devices is because the TalkTalk Mail system will detect and flag as suspicious geographically distanced access points used by different devices. For example if you're using a connection via your home broadband and there's also an unauthorised device connecting from a different country or you have another device connecting via an overseas hosted VPN server then these can trigger rate limiting.
