If nothing is helping and you've made sure that
all of your email account parameters are set to what they should be according to what is required by your ISP and
after trying the "Repair" option in your MS Outlook client program ("File" > "Account Settings" > "Repair" for each account in Outlook) and you still get errors then you could try the following (I'm not sure if this will help but it is a relatively benign application of this utility so it's worth a try):
Open the
Microsoft Exchange Server Administrator Utility (click on link for details on this utility).
Click on the “Connections” icon on the left side of the navigational panel for the server you want to administer.
Double click on the “Internet Mail Service” icon on the right side of the panel to open up the “Properties” window.
Again, select the “Connections” tab from the properties window and click on the “Advanced” option.
This will open up a configuration window on your screen.
Now by choosing the “Maximum no. of inbound connections” option, you can set the same to a higher number than what is already available. Keep resetting the numbers higher than the first until you find that the errors are resolved.
At the point errors are resolved then you can close all open Exchange Server Administrator program windows to complete the process.