POP3 Login Fails for Exchange Server 2007 Users With Long UPN

Source: Microsoft Support

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Users try to connect to their mailboxes on Exchange Server 2007 via POP3 connection.

Some of them may fail to log in and receive the following error message:

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

This problem occurs because Exchange Server 2007 has its “MaxCommandSize” setting for POP3 configured to use the value "40" by default. This limits the maximum size of a single command to 40 bytes. Some users with long UPN (like testuserwithverylongname@myemaildomainame.com) may fail to connect due to this limit.

To resolve this problem, use either of the follow methods:

Method 1: Increase the value of MaxCommandSize from Exchange Management Shell

1. Click Start, click All Programs, click Microsoft Exchange Server 2007 and click Exchange Management Shell.
2. Run the following cmdlet to verify the current value of MaxCommandSize:
Get-POPSettings
In the output, the value of MaxCommandSize is 40 by default.
3. Run the following cmdlet to increase the value of MaxCommandSize:
Set-POPSettings -MaxCommandSize 46
In this example, we increased it to 46. The possible values are from 40 to 1024 (bytes).
4. Restart the Microsoft Exchange POP3 service.

Method 2: Increase the value of MSExchPopImapCommandSize by using the Active Directory Service Interfaces (ADSI) Edit snap-in.

To do this, follow these steps:

Note: ADSI Edit is included with the Microsoft Windows Server 2003 Support Tools. To install the Windows Server 2003 Support Tools, double-click Suptools.msi in the Support\Tools folder on the Windows Server 2003 CD.

Warning: If you use the ADSI Edit snap-in, the LDP utility, or any other LDAP version 3 client, and you incorrectly modify the attributes of Active Directory objects, you can cause serious problems. These problems may require you to reinstall Microsoft Windows 2000 Server, Microsoft Windows Server 2003, Microsoft Exchange 2000 Server, Microsoft Exchange Server 2003, Microsoft Exchange Server 2007 or both Windows and Exchange. Microsoft cannot guarantee that problems that occur if you incorrectly modify Active Directory object attributes can be solved. Modify these attributes at your own risk.

1.Click Start, point to All Programs, point to Windows Support Tools, and then click Command Prompt.
2.At the command prompt, type adsiedit.msc , and then press ENTER.
3.Connect to a domain controller if you are not already connected.
4.Expand Configuration Container [ computername.contoso.com ], expand CN=Configuration,DC= contoso ,DC=com, expand CN=Services, expand CN=Microsoft Exchange, expand CN=Administrative Groups, expand CN=Exchange Administrative Groups, expand CN=<your serer name>, expand CN=Services Protocols, expand CN=POP3, right-click CN=1, and then click Properties.
Note In this step, contoso.com is a placeholder for the domain name. The other words in italic are also placeholders.
5.In the CN=1 Properties dialog box, locate and then double-click the MSExchPopImapCommandSize attribute.
6.Change the value of MSExchPopImapCommandSize attribute to 46, and then click OK to close ADSI Edit.
Note In this example, we increased it to 46. The possible values are from 40 to 1024 (bytes).
7.Restart the Microsoft Exchange POP3 service.

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