Below is a detailed list of every error code ever discovered. Some of the commands have never before been encountered, but exist nonetheless. Each error code has details on every way I have seen them used, but it is quite possible that there are missing situations.

200's

200: Invalid syntax
Never before encountered.
201: Invalid parameter
Sent if the command has a parameter that is considered invalid by the server. Setting your status to FLN would be an example.
    >>> CHG 19 FLN\r\n
    <<< 201 19\r\n
Also sent if you try to ADD an invalid email address.
    >>> ADD 20 FL a@b a@b\r\n
    <<< 201 20\r\n
205: Invalid user
Sent if you try to ADD a nonexistent, but valid, account name.
>>> ADD 21 AL non_existent_account@passport.com non_existent_account@passport.com\r\n
<<< 205 21\r\n
206: Domain name missing
Never before encountered.
207: Already logged in
If the client sends a USR after already logging in, this error will be sent in response.
    >>> USR 20 MD5 I example@passport.com\r\n
    <<< 207 20\r\n
    >>> USR 21 MD5 S 6b05a0f5b1aa0959e128cfdd513ccf5c\r\n
    <<< 207 21\r\n
208: Invalid username
Sent from the switchboard in reply to a CAL which specifies an invalid account name.
>>> CAL 2 @@a\r\n
<<< 208 2\r\n
209: Nickname change illegal
Sent in reply to a REA when there is a blocked word/phrase in the new nickname or when the Passport has not yet been confirmed.
>>> REA 18 user@passport.com look%20msn\r\n
<<< 209 18\r\n
210: User list full
Sent in response to an ADD command if the FL is already full. The user will not be added. Currently (as of March 23, 2003), the limit is 150 users, as doubled from a previous 75.
>>> ADD 23 FL user@passport.com user@passport.com 0\r\n
<<< 210 23\r\n
215: User already on list
Sent if you try to ADD a user to a list, but the user is already in that list.
    >>> ADD 26 AL user@passport.com user@passport.com\r\n
    <<< ADD 26 AL 824 user@passport.com user@passport.com\r\n
    >>> ADD 27 AL user@passport.com user@passport.com\r\n
    <<< 215 27\r\n
Also sent if you try to ADD a user to your FL in a group that the user is already in.
    >>> ADD 28 FL user@passport.com user@passport.com 2\r\n
    <<< ADD 28 825 FL user@passport.com user@passport.com 2\r\n
    >>> ADD 29 FL user@passport.com user@passport.com 2\r\n
    <<< 215 29\r\n
Also sent if you try to invite (CAL) a user into a switchboard session that has already been invited and/or is already present.
    >>> CAL 5 example@passport.com\r\n
    <<< CAL 5 RINGING 286195\r\n
    >>> CAL 6 example@passport.com\r\n
    <<< 215 6\r\n
216: User not on list
Sent in reply to REMs that try to remove users that are not in the list. Also sent if you try to remove nonexistent or invalid account names because, naturally, they couldn't be in your list.
    >>> REM 14 FL user_not_in_list@msn.com\r\n
    <<< 216 14\r\n
This error is also sent in reply to invitations to the switchboard (CALs) to users that either have you in their BL or have their BLP settings so that you are automatically in their BL. This is only sent when the user is online, but you're not allowed to know their online status.
    >>> CAL 2 example@passport.com\r\n
    <<< 216 2\r\n
Also sent in reply to REAs that try to rename users that are not in any of your lists.
    >>> REA 48 random_user@randomdomain.com nickname\r\n
    <<< 216 48\r\n
217: User not online
This error is sent in reply to invitations to the switchboard (CALs) to users that are currently either offline or appearing offline. This error will also be sent in reply to invitations to nonexistent account names.
    >>> CAL 2 myname@msn.com\r\n
    <<< 217 2\r\n
    >>> CAL 3 nonexistent@passport.com\r\n
    <<< 217 3\r\n
218: Already in mode
Sent in reply to a GTC or a BLP if it tries to change the mode to the mode it is already in. In other words, if the GTC value is already set to A, and the client sends the GTC command setting it to A again, it will receive this error.
    >>> GTC 15 A\r\n
    <<< GTC 15 150 A\r\n
    >>> GTC 16 A\r\n
    <<< 218 16\r\n

    >>> BLP 17 BL\r\n
    <<< BLP 17 151 BL\r\n
    >>> BLP 18 BL\r\n
    <<< 218 18\r\n
219: User is in the opposite list
Send in reply to an ADD that causes the same user to be in the BL and the AL at the same time. Obviously, that cannot be possible, and the user will not be added.
    >>> ADD 15 AL example@passport.com example@passport.com\r\n
    <<< ADD 15 AL 108 example@passport.com Mike\r\n
    >>> ADD 16 BL example@passport.com example@passport.com\r\n
    <<< 219 16\r\n
223: Too many groups
Sent if you try to add a group (ADG) so that you have more than 30 groups (the maximum). The group with the ID of zero counts toward the limit.
>>> ADG 38 thirtyfirst%20group 0\r\n
<<< 223 38\r\n
224: Invalid group
Sent if you try to ADD a user to your FL with a nonexistent group ID. Also sent in reply to REMs to nonexistent groups. Also sent in reply if you try to remove (RMG) or rename (REG) a nonexistent group.
    >>> RMG 24 15\r\n
    <<< RMG 24 12065 15\r\n
    >>> ADD 25 FL example@passport.com example@passport.com 15\r\n
    <<< 224 25\r\n
    >>> REM 26 FL example@passport.com 15\r\n
    <<< 224 26\r\n
    >>> RMG 27 15\r\n
    <<< 224 27\r\n
    >>> REG 28 15 New%20Name 0\r\n
    <<< 224 28\r\n
225: User not in group
Sent if you try to remove (REM) a user from an existent group that they are not currently in.
>>> REM 27 FL example@passport.com 3\r\n
<<< 224 27\r\n
229: Group name too long
Sent if you try to add a group (ADG) with more than 61 bytes in the name.
>>> ADG 50 this%20group's%20name%20is%20sixty%20two%20bytes%20in%20length 0\r\n
<<< 229 50\r\n
230: Cannot remove group zero
Sent if you try to remove (RMG) the group with the ID of zero.
>>> RMG 28 0\r\n
<<< 230 28\r\n
231: Invalid group
Never before encountered.
280: Switchboard failed
Never before encountered.
281: Transfer to switchboard failed
Never before encountered.

300's

300: Required field missing
Never before encountered.
301: Too many hits to a FND
Never before encountered.
>>> FND 103 fname=Fred lname=Smith city=* state=* country=*\r\n
<<< 301 103\r\n
302: Not logged in
Never before encountered.

500's

500: Internal server error
When the MSN servers are down, this might be sent in reply to your command to the NS containing your digested password. The official client interprets this as "Service Temporarily Unavailable". This error might also be sent if your account has banned from MSN.
>>> USR 28 MD5 S 1938acd78b5524ce56a0ab6a01f3239\r\n
<<< 500 28\r\n
501: Database server error
Never before encountered.
502: Command disabled
Sent if you try to use a command associated with an MSN Messenger feature that no longer exists.
>>> FND 3 fname=Bill lname=Gates city=* state=* country=US\r\n
<<< 502 3\r\n
510: File operation failed
Never before encountered.
520: Memory allocation failed
Never before encountered.
540: Challenge response failed
Sent in response to a QRY that is invalid. The server will close the connection after sending the error.
    <<< CHL 0 15570131571988941333\r\n
    >>> QRY 10 msmsgs@msnmsgr.com 32\r\n         4f2f5a91b72102cd28355e9fc9000d6e
    <<< 540 10\r\n

600's

600: Server is busy
Never before encountered.
601: Server is unavailable
Can be sent in reply to the initial USR to the NS if the server is somehow "unavailable".
>>> USR 10 MD5 I example@passport.com\r\n
<<< 601 10\r\n
602: Peer nameserver is down
Never before encountered.
603: Database connection failed
Never before encountered.
604: Server is going down
Never before encountered.
605: Server unavailable
Never before encountered.

700's

707: Could not create connection
Never before encountered.
710: Bad CVR parameters sent
Sent in reply to a CVR if you send an invalid LCID.
>>> CVR 1 0x0809 winnt 5.1 i386 MSNMSGR 6.0.0602 MSMSGS example@passport.com\r\n
<<< 710 1\r\n
711: Write is blocking
Never before encountered.
712: Session is overloaded
Never before encountered.
713: Too many active users
Never before encountered.
714: Too many sessions
Never before encountered.
715: Not expected
Sent in response to a PRP setting an invalid phone type of three or less characters.
    >>> PRP 30 PHV 555-0690\r\n
    <<< 715 30\r\n
Also sent in response to a change of friendly name (REA) on an unverified Passport account.
    >>> REA 31 me@passport.com newname\r\n
    <<< 715 31\r\n
717: Bad friend file
Never before encountered.
731: Not expected
Sent in response to a badly formatted CVR     >>> VER 1 MSNP7\r\n
    >>> CVR 2 foo\r\n
    <<< VER 1 MSNP7\r\n
    <<< 731 2\r\n
    <o> Server Closes Connection\r\n

800's

800: Friendly name changes too rapidly
When the client tries to change the friendly name (REA) too rapidly, this error message will be sent in response to the commands that the server rejects. This does not affect renaming other users.
    >>> REA 100 me@passport.com NAME\r\n
    <<< REA 100 2000 me@passport.com NAME\r\n
    >>> REA 101 me@passport.com NAME\r\n
    <<< REA 101 2001 me@passport.com NAME\r\n
    >>> REA 102 me@passport.com NAME\r\n
    <<< REA 102 2002 me@passport.com NAME\r\n
    >>> REA 103 me@passport.com NAME\r\n
    <<< REA 103 2003 me@passport.com NAME\r\n
    >>> REA 104 me@passport.com NAME\r\n
    <<< 800 104\r\n
Also when the client tries to change the status (CHG) too rapidly.
    >>> CHG 105 NLN\r\n
    <<< CHG 105 NLN\r\n
    >>> CHG 106 HDN\r\n
    <<< CHG 106 HDN\r\n
    >>> CHG 107 NLN\r\n
    <<< CHG 107 NLN\r\n
    >>> CHG 108 HDN\r\n
    <<< CHG 108 HDN\r\n
    >>> CHG 109 NLN\r\n
    <<< 800 109\r\n

900's

910: Server too busy
When the MSN servers are busy, this might be sent in reply to your command to the NS containing your digested password..
>>> USR 28 MD5 S 1938acd78b5524ce56a0ab6a01f3239\r\n
<<< 910 28\r\n
911: Authentication failed
Sent in response to some invalid formats of the USR command to the DS. Also sent if the account name is invalid.
    >>> USR 3 MD5 S example@passport.com\r\n
    <<< 911 3\r\n

    >>> USR 4 MD5 I passport.com\r\n
    <<< 911 4\r\n
Sent in response to an invalid MD5 digest sent in the second USR command to the NS. Also sent if the account name is invalid or nonexistent.
    >>> USR 6 MD5 S 23e54a439a6a17d15025f4c6cbd0f6b4\r\n
    <<< 911 6\r\n
Also sent in response to problems authenticating with the switchboard with USR or ANS.
    >>> USR 1 myname_123@hotmail.com 486979.1047702516.105\r\n
    <<< 911 1\r\n
    >>> ANS 1 myname_123@hotmail.com 849102291.520491932 11752098\r\n
    <<< 911 1\r\n
912: Server too busy
Never before encountered.
913: Not allowed when offline
Sent in reply to commands that are not allowed in the HDN status which the user is currently in. The only known command that is prohibited when offline is XFR
>>> XFR 13 SB\r\n
<<< 913 13\r\n
914: Server unavailable
Never before encountered.
915: Server unavailable
Never before encountered.
916: Server unavailable
Never before encountered.
917: Authentication failed
Never before encountered.
918: Server too busy
Never before encountered.
919: Server too busy
Never before encountered.
920: Not accepting new users
Never before encountered.
921: Server too busy
When the MSN servers are busy, this might be sent in reply to your command to the NS containing your digested password..
>>> USR 28 MD5 S 1938acd78b5524ce56a0ab6a01f3239\r\n
<<< 921 28\r\n
922: Server too busy
Never before encountered.
923: Kids Passport without parental consent
Sent in reply to the second USR to the NS if client is logging in with a Kids Passport without parental consent to chat online.
>>> USR 29 MD5 S 3b18acd78b5524ce56a0ab6a01f3239\r\n
<<< 923 29\r\n
924: Passport account not yet verified
Never before encountered.