Question : POP3 server unavailable ERR 21

I run Exchange 2008 on my SBS 2008 server.  I use POP3 to log in remotely to check my email with Outlook 2010.

All has been running well and then all of a sudden without touching anything on the server or the netbook Outlook fails to connect to the SBS2008 Exchange server and returns the following message after asking my for my user and password:-
! Task 'Home POP3 Server - Receiving' reported error (0x800CCC92): Your e-mail server rejected your login.  Verify your user name and password for this account in Accounting Settings.  The server responded : -ERR Server Unavailable. 21'

"Home POP3 Server' is the name of my account.

So I have tried TELNET ie
TELNET
OPEN server 110
and then I submit by user code and password with USER and PASS COMMANDS and it responds with the following messages:-
-ERR Server Unavailable. 21
-ERR Connection is closed. 22

I have tried restarting the Server - still there.
I have looked at the services running - everything seems to be fine.

Any ideas????

Answer : POP3 server unavailable ERR 21

Well I have solved it.
I went back through the event log on the server to see what had happened between when my POP3 access to the email server worked and when it didn't.  And I found that my server had a hard disk error apparently on the Mailbox!!

I ran CHKDSK /R and rebooted the server.

This seems to have fixed everything.  No sign of any ill effects on the mailbox or loggin in to the POP3 server at all.
Peter