Except 226 and 426 aren't allowed response to a MKD according to the RFC.
http://www.w3.org/Protocols/rfc959/9_References.html
Check 4.2. FTP REPLIES
5yz Permanent Negative Completion reply
...
x5z File system - These replies indicate the status of the
Server file system vis-a-vis the requested transfer or
other file system action.
...