The following chart contains a list of possible error codes that may be returned by the Microsoft mail clients listed at the beginning of this article. This  list of error codes may be returned when a failure occurs sending and receiving mail.
Click here to read about the differences between MS Outlook and MS Outlook Express.
MS Outlook/Express Error Codes
Error Code     Error Type
0x800CCC00  LOAD_SICILY_FAILED  Authentication did not load.
0x800CCC01  INVALID_CERT_CN  Invalid certificate content
0x800CCC02  INVALID_CERT_DATE  Invalid certificate date.
0x800CCC03  ALREADY_CONNECTED  User already connected.
0x800CCC04  CONN   
0x800CCC05  NOT_CONNECTED  Not connected to server.
0x800CCC06  CONN_SEND   
0x800CCC07  WOULD_BLOCK   
0x800CCC08  INVALID_STATE   
0x800CCC09  CONN_RECV   
0x800CCC0A  INCOMPLETE  Message download incomplete.
0x800CCC0B  BUSY  Server or maildrop is busy.
0x800CCC0C  NOT_INIT   
0x800CCC0D  CANT_FIND_HOST  Cannot locate server.** Server is down**
0x800CCC0E  FAILED_TO_CONNECT  Cannot connect to server.
0x800CCC0F  CONNECTION_DROPPED  Connection closed.
0x800CCC10  INVALID_ADDRESS  Address not known on server.
0x800CCC11  INVALID_ADDRESS_LIST  Mailing list not known on server.
0x800CCC12  SOCKET_READ_ERROR  Unable to send Winsock request.
0x800CCC13  SOCKET_WRITE_ERROR  Unable to read Winsock reply.
0x800CCC14  SOCKET_INIT_ERROR  Unable to initialize Winsock.
0x800CCC15  SOCKET_CONNECT_ERROR  Unable to open Windows Socket.
0x800CCC16  INVALID_ACCOUNT  User account not recognized.
0x800CCC17  USER_CANCEL  User canceled operation.
0x800CCC18  SICILY_LOGON_FAILED  Logon attempt failed.
0x800CCC19  TIMEOUT    
0x800CCC1A  SECURE_CONNECT_FAILED  Unable to connect using SSL.
0x800CCC40  WINSOCK_WSASYSNOTREADY  Network subsystem is unusable.
0x800CCC41  WINSOCK_WSAVERNOTSUPPORTED  Windows Sockets cannot support this application.
0x800CCC42  WINSOCK_WSAEPROCLIM    
0x800CCC43  WINSOCK_WSAEFAULT  Bad address.
0x800CCC44  WINSOCK_FAILED_WSASTARTUP  Unable to load Windows Sockets.
0x800CCC45  WINSOCK_WSAEINPROGRESS  Operation now in progress. This error appears if a Windows Sockets API is called while a blocking function is in progress.
0x800CCC60  SMTP_RESPONSE_ERROR  Invalid response.
0x800CCC61  SMTP_UNKNOWN_RESPONSE_CODE  Unknown error code.
0x800CCC62  SMTP_500_SYNTAX_ERROR  Syntax error returned.
0x800CCC63  SMTP_501_PARAM_SYNTAX  Parameter syntax incorrect.
0x800CCC64  SMTP_502_COMMAND_NOTIMPL  Command not implemented.
0x800CCC65  SMTP_503_COMMAND_SEQ  Improper command sequence.
0x800CCC66  SMTP_504_COMMAND_PARAM_NOTIMPL  Command not implemented.
0x800CCC67  SMTP_421_NOT_AVAILABLE  Command not available.
0x800CCC68  SMTP_450_MAILBOX_BUSY  Mailbox is locked and busy.
0x800CCC69  SMTP_550_MAILBOX_NOT_FOUND  Mailbox not found.
0x800CCC6A  SMTP_451_ERROR_PROCESSING  Error processing request.
0x800CCC6B  SMTP_551_USER_NOT_LOCAL  User mailbox is known, but mailbox not on this server.
0x800CCC6C  SMTP_452_NO_SYSTEM_STORAGE  No space to store messages.
0x800CCC6D  SMTP_552_STORAGE_OVERFLOW  Storage limit exceeded.
0x800CCC6E  SMTP_553_MAILBOX_NAME_SYNTAX  Invalid mailbox name syntax.
0x800CCC6F  SMTP_554_TRANSACT_FAILED  Transaction failed.
0x800CCC78  SMTP_REJECTED_SENDER  This is caused by having the incorrect e-mail address in the Reply-To field.
0x800CCC79  SMTP_REJECTED_RECIPIENTS  Server rejected recipients.
0x800CCC7A  SMTP_NO_SENDER  No sender address specified.
0x800CCC7B  SMTP_NO_RECIPIENTS   No recipients specified. 
0x800420CB  POP3_NO_STORE  Mail cannot be stored on server
0x800CCC90  POP3_RESPONSE_ERROR   Client response invalid.
0x800CCC91  POP3_INVALID_USER_NAME  Invalid user name or user not found.
0x800CCC92  POP3_INVALID_PASSWORD  Password not valid for account.
0x800CCC93  POP3_PARSE_FAILURE  Unable to interpret response.
0x800CCC94  POP3_NEED_STAT STAT  Command required.
0x800CCC95  POP3_NO_MESSAGES  No messages on server.
0x800CCC96  POP3_NO_MARKED_MESSAGES  No messages marked for retrieval.
0x800CCC97  POP3_POPID_OUT_OF_RANGE  Message ID out of range.
0x800CCCA0  NNTP_RESPONSE_ERROR  News server response error.
0x800CCCA1  NNTP_NEWGROUPS_FAILED  Newsgroup access failed.
0x800CCCA2  NNTP_LIST_FAILED LIST  command to server failed.
0x800CCCA3  NNTP_LISTGROUP_FAILED  Unable to display list.
0x800CCCA4  NNTP_GROUP_FAILED  Unable to open group.
0x800CCCA5  NNTP_GROUP_NOTFOUND  Group not on server.
0x800CCCA6  NNTP_ARTICLE_FAILED  Message not on server.
0x800CCCA7  NNTP_HEAD_FAILED  Message header not found.
0x800CCCA8  NNTP_BODY_FAILED  Message body not found.
0x800CCCA9  NNTP_POST_FAILED  Unable to post to server.
0x800CCCAA  NNTP_NEXT_FAILED  Unable to open next message.
0x800CCCAB  NNTP_DATE_FAILED  Unable to display date.
0x800CCCAC  NNTP_HEADERS_FAILED  Unable to display headers.
0x800CCCAD  NNTP_XHDR_FAILED  Unable to display MIME headers.
0x800CCCAE  NNTP_INVALID_USERPASS  Invalid user or password. 
0x800CCCC2  RAS_NOT_INSTALLED  RAS/DUN not installed.
0x800CCCC3  RAS_PROCS_NOT_FOUND  RAS/DUN process not found. 
0x800CCCC4  RAS_ERROR   RAS/DUN error returned.
0x800CCCC5  RAS_INVALID_CONNECTOID  Connectoid damaged or missing.
0x800CCCC6  RAS_GET_DIAL_PARAMS  Error getting dial settings
0x800CCCD1  IMAP_LOGINFAILURE  Login failed.
0x800CCCD2  IMAP_TAGGED_NO_RESPONSE   Message tagged.
0x800CCCD3  IMAP_BAD_RESPONSE  Invalid response to request.
0x800CCCD4  IMAP_SVR_SYNTAXERR  Syntax error.
0x800CCCD5  IMAP_NOTIMAPSERVER  Not an IMAP server.
0x800CCCD6  IMAP_BUFFER_OVERFLOW  Buffer limit exceeded.
0x800CCCD7  IMAP_RECVR_ERROR  Recovery error.
0x800CCCD8  IMAP_INCOMPLETE_LINE  Incomplete data.
0x800CCCD9  IMAP_CONNECTION_REFUSED  Connection not allowed.
0x800CCCDA  IMAP_UNRECOGNIZED_RESP  Unknown response.
0x800CCCDB  IMAP_CHANGEDUID  User ID has changed.
0x800CCCDC  IMAP_UIDORDER  User ID command failed.
0x800CCCDD  IMAP_UNSOLICITED_BYE  Unexpected disconnect.
0x800CCCDE  IMAP_IMPROPER_SVRSTATE  Invalid server state.
0x800CCCDF  IMAP_AUTH_NOT_POSSIBLE  Unable to authorize client.
0x800CCCE0  IMAP_OUT_OF_AUTH_METHODS  No more authorization types.
0x80042108   OL: Error Message: 0x80042108 Outlook Is Unable to Connect to Your Incoming POP3 E-mail Server
Reference: http://servertune.com/kbase/entry/42/
