Unable to FTP due to SIZE subcommand not valid

Question

We are trying to FTP from a known outside server to an inside machine. Both are IBM i servers. The connection comes thru the firewall and authenticates, but transfer fails. We are using BIN and PASSIVE. It appears the XG-310 is trying to pre-download and using SIZE to determine the download, but SIZE is not supported on either FTP server or client. 
 This procedure has been working for years until the XG-310 was installed. Is there a way to bypass the Sophos inrecept and let it download? FTP files are only from one server, and are always small. Traffic is sending files from outside to inside on port 23.

Terence Barstow · Answer

Thanks for the suggestions. It did not fix it, but it got me looking in the right direction. Utimately I found it was LAN to WAN tranfser. I found I was able to edit the rule to uncheck scan FTP for malware. That is what I suspected, I just couldn't find the setting. (Of course I was looking at incoming rules instead of outgoing rules, too.)