This article contains a list of possible SMTP & POP error codes that may be returned by the Microsoft mail clients when trying to send/receive email:

Simple Mail Transport Protocol (SMTP) Errors
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ERROR CODE DESCRIPTION
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0x800CCC60 Invalid response.
0x800CCC61 Unknown error code.
0x800CCC62 Syntax error returned.
0x800CCC63 Parameter syntax incorrect.
0x800CCC64 Command not implemented.
0x800CCC65 Improper command sequence.
0x800CCC66 Command not implemented.
0x800CCC67 Command not available.
0x800CCC68 Mailbox is locked and busy.
0x800CCC69 Mailbox not found.
0x800CCC6A Error processing request.
0x800CCC6B User mailbox is known, but mailbox not on this server.
0x800CCC6C No space to store messages.
0x800CCC6D Storage limit exceeded.
0x800CCC6E Invalid mailbox name syntax.
0x800CCC6F Transaction failed.
0x800CCC78 Unknown sender. This is caused by having the incorrect e-mail address in the Reply-To field.
0x800CCC79 Server rejected recipients.
0x800CCC7A No sender address specified.
0x800CCC7B No recipients specified.

Post Office Protocol version 3 (POP3) Errors
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ERROR CODE DESCRIPTION
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0x800420CB Mail cannot be stored on server.
0x800CCC90 Client response invalid.
0x800CCC91 Invalid user name or user not found.
0x800CCC92 Password not valid for account.
0x800CCC93 Unable to interpret response.
0x800CCC94 STAT Command required.
0x800CCC95 No messages on server.
0x800CCC96 No messages marked for retrieval.
0x800CCC97 Message ID out of range.