Hi there. TESCO.NET host. I spoke with them at length but - you guessed - they said it was a Microsoft update problem.........................
And let me also guess - they didn't happen to mention "which" Microsoft update caused the problem so have to wonder how they came to that conclusion.
Anyhow - note the bold part in the error you've encountered - which is an error being returned by your ISP's SMTP server and has absolutely nothing to do with Outlook
Sending reports error (0x 800CCC78) Unable to send the message. Please verify the e-mail address in your account properties. The server responded 452 4.7.0 (F11)
To many messages sent.
Virtually every ISP these days is enforcing some kind of limit for messages sent which could be impacting the issue. The controls can be any combination of the following
#1 - # of recipients in one message
#2 - total # of messages sent in any given hour
#3 - total # messages sent in any given day