If Windows Live Messenger won’t let you log in, there are a number of checks you need to follow to solve the problem. Starting with the easiest, this walkthrough will help…
There might be a problem with your Internet connection. Make sure your computer is connected to the Internet. Click Troubleshoot to try and fix this problem.
The trouble is after having tried the troubleshooter everything reports back as completely fine as you can see from the screenshot above, so what gives?
Looking on the official Windows Live Messenger website they suggest that one downloads the very latest version of Live Messenger! Now I know my version of Messenger was logging in a few weeks ago, but hasn’t been able to since.
Updateing Messenger, did seem like a sensible suggestion, so fair enough. The download may have taken ages but if that fixed the problem it would all be worth it.
…half an hour later.. Tadaaa! Nothing. Same problem same error 800727475.
However Messenger did come back with a suggestion:
Windows Live Messenger has made several failed attempts to sign you in. (No kidding!) Your firewall may be blocking Windows Live Messenger from connecting to the service. Please review your firewall setings…
Well seeing as all my routers outbound ports are unblocked, I really don’t see how it can be.
It turns out that it is a router problem, you just need to find the Maximum Transmit Units (MTU) and change it to 1400 and everything should work just fine…