The sending is being blocked as a security measure because the system is detecting multiple concurrent sign ins to your mailbox but from different locations.
How do you normally use email? With or without VPN?
To link this topic to your TalkTalk service please check your Community Profile includes in Personal Information (Click here):
Your name, current TalkTalk landline 'phone and alternate number (mobile preferred). Add full address with postcode (location). Then scroll down to Private notes to add your email address, notes and references etc and then save changes.
If your mailbox is on the TalkTalk Mail platform then compose and send the same mailbox a message to see if that sends without error, is copied to the Sent objects folder and then arrives and stays in the Inbox.
If the send fails then let us know what the full error message is.
Message could not be sent to the following recipients: [****t] (453 - 453 4.2.0 8LN8ke5qMvqfm Too many messages (TT602)
I tried sending a message to myself, this is the message I got, I am nowhere near my quota
I am working on a pc but also check my emails on my phone and on a galaxy tab
The error message is also indicative of email being rate limited possibly due to multiple concurrent sign ins from different locations. You may need to wait a few hours for the rate limiting to be reset.
Are you sending a lot of mail messages?
To get you back in action and to lock out any unauthorised access to your mailbox it's a good idea to change your mailbox password to an ultra-strong new password and then just sign in to TalkTalk Mail but for the moment don't change the password for the other devices.