9/17/2008 -- This type of error typically results when users have a long user principal name (UPN) that exceeds the default maximum of 40 characters. Because UPN includes the user's log-on name, the @ symbol, and the domain name, if the MaxCommand Size setting for POP3 exceeds 40 bytes, you're likely to see the following error:

There was a problem logging onto your mail server. Your User Name was rejected.
Account: "Account name",
Server: "Server name",
Protocol: POP3,
Server Response: "-ERR Protocol error. Connection is Closed."
Port: 110, Secure (SSL): No, Server Error: 0x800CCC90, Error Number: 0x800CCC91

There are a couple of ways to resolve this issue. You can either increase the value of MaxCommandSize as described below, or increase the value of MSExchPopImapCommandSize by using the adsiedit.msc snap-in.

To change the MaxCommandSize value, do the following:

1. Start the Exchange Management Shell.
2. Type "Get-POPSettings" (without the quotes) to view current settings. Notice that the default value of MaxCommandSize is set to 40.
3. To increase the value of MaxCommandSize from the default 40 to, say, 50, type "Set-POPSettings -MaxCommandSize 50" (without the quotes). You can use any value between 40 and 1,024 bytes.
4. Restart the POP3 service for the changes to take effect.

To change the MSExchPopImapCommandSize, check out the KB article 945552.