I also cannot send email using either webmail or Outlook
Its not the first time it has happened over the years, cleraly some underlying design or build fault.
It usually gets sorted out in a week or two, still a real pain though, I am looking at jacking in talk talk altogether and getting something reliable.
Come on Talk Talk find out what the problem is and FIX IT once and for all.
I have the same problem now. I am unable to send Lineone emails from Outlook today, even though I can receive them.
No settings have been changed and I have been using Outlook on this PC for over 4 years without issues.
Webmail sends fine.
Test email from Outlook fails
Do you have another SMTP that I can try?