Flow: MDTM comes in two forms. With a single argument it reports the timestamp of a file. With two arguments it can be used to modify the timestamp of the file. This is really useful for mirroring servers.
The 501 error for MDTM is ioFTPD replying that it didn't like the arguments. I don't know which form of the command the client was trying to issue... The exact form of the arguments for modifying the timestamp can be seen in the FEAT response. Check to make sure it matches.
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