POP3 Server Unavailable. 21

Recently I entered the dreaded -ERR Server Unavailable. 21 error on an Exchange Server. The strange this was that this did not happen to all mailboxes but only to a couple of mailboxes. Nothing written in the eventlog, just –ERR Server Unavailable. 21 immediately after entering my credentials (this error can be seen when using TELNET to access a mailbox using POP3).

To get more information regarding the login process you can enable POP logging on the CAS Server. Be very careful with this and only enable it for test purposes. If you forget to turn it off most likely your CAS server will shutdown in a couple of days because the system drive is filled up with POP3 log files.

Enable POP logging using the Exchange Management Shell:

Set-POPSettings –ProtocolLogEnabled:$TRUE

And don’t forget to restart the POP3 service.

The POP3 log files are stored in C:\Program Files\Microsoft\Exchange Server\V14\Logging\Pop3. After testing and the Error Server Unavailable. 21 error show up, disable logging (Set-POPSettings –ProtocolLogEnabled:$FALSE), restart the service and open the POP3 log file.

2012-03-08T21:26:02.937Z,0000000000000002,1,, ,,0,23,5,user,joe@sixpack.com,R=ok;RpcL=-1;LdapL=-1
2012-03-08T21:26:13.296Z,0000000000000002,2,, ,,46,10,29,pass,*****,"R=""-ERR Server Unavailable. 21"";RpcL=-1;LdapL=-1;
Msg=User:joe:bf6c873e-4289-4a64-87bf-2ade894947cd:DAG04-MBX05:MBX19.E14.local; Excpt=""Could not load file or assembly ‘Microsoft.Exchange.Compliance, Version=, Culture=neutral, PublicKeyToken=31bf3856ad364e35’ or one of its dependencies. The system cannot find the file specified.-FileNotFoundException"""

This turns out to be a bug in the Exchange 2010 setup (even in SP1) where the POP3 config file is not created appropriately. Unfortunately the same is true for IMAP4, but the fix is identical. The config files are called Microsoft.Exchange.Pop3.exe.config and Microsoft.Exchange.Imap4.exe.config and can be found in the directory C:\program files\Microsoft\Exchange Server\V14\ClientAccess\PopImap\

Add the following pieces to the XML file (both POP3 and IMAP4):

<assemblyIdentity name="Microsoft.Exchange.Compliance" publicKeyToken="31bf3856ad364e35" culture="neutral" />
<codeBase version="" href="file:///C:\Program Files\Microsoft\Exchange Server\V14\bin\Microsoft.Exchange.Compliance.dll" />


Restart the POP3 and IMAP4 service and the POP3 Server Unavailable no longer appears.

Telnet for POP3 and IMAP4

For testing purposes it is useful to know how to use TELNET to access a mailbox using the POP3 or the IMAP4 protocol. Suppose you have a mail server running with POP3 and IMAP4 enabled and you want to give it a try.


On a client open a command prompt and enter TELNET <<mailserver>> 110

If the server is running fine you’ll see a welcome message (like +OK The Microsoft Exchange POP3 service is ready) and you can enter the user credentials:

User <<username>>

Pass <<password>>


You are now successfully logged on. If you enter the LIST command all items in the Inbox are shown. RETR 1 will retrieve the first message in this list while DELE 1 will delete this first message. Bottom line is that you are successfully logged on, POP3 is running fine and the mailbox can be accessed.

Use the QUIT command to log off from the mailbox.


On a client computer open a command prompt and enter TELNET <<mailserver>> 143

If the server is running fine you’ll see a welcome message (like * OK The Microsoft Exchange IMAP4 service is ready) and you can enter the user credentials:

. login <<username>> <<password>>

That is: dot space login <<username>> <<password>>, you have to enter the dot and the space before the login command. If you omit either one the login will fail!


To get an overview of all the folders in the inbox you can use the LIST command:

. LIST “” “*”

Again, you have to enter the dot and the space before the actual LIST command.

Use the STATUS command to get the number of messages, for example:

. status inbox (messages)


Logoff from an IMAP4 session is a bit peculiar, use the CTRL + ] key combination, followed by the QUIT command (to actually quit the TELNET session).


Useful commands for a quick test of your mailbox and the POP3 and IMAP4 services.

If you want a complete overview of all commands please check this websites:

http://bobpeers.com/technical/telnet_imap and http://bobpeers.com/technical/telnet_pop