TT608 means that the TalkTalk Mail platform has detected multiple logins to your mailbox and such that the system thinks that your password has been discovered. As a result the mailbox has been restricted and a limit put on sending mail.
As a precaution in the case of a compromised password you should change and upgrade the password to lock out any unauthorised sign ins. The restriction on sending may last for up to 24 hours.
Did you have multiple devices signed in and are any using a VPN, perhaps as part of security software like Norton or AVG, for example? In the event that you were using a VPN and making multiple concurrent logins yourself then make sure the VPN is set to use a UK host server. Try to avoid having sign ins from multiple geographic locations e.g. from your home and at the same time from your work based VPN.
