If that server is configured to disallow fragmented packets it will send the ''Fragmentation needed but disallowed, packet discarded'' message back to you. So could any other hop on the route, of course. It's pretty normal TCP/IP protocol stuff and FileZilla knows how to handle it. However, the message packets have to reach you or the MTU path discovery fails. ICMP packets should be allowed to pass, especially if you want to be prepared for what lies before us: IPv6 does not allow packets to be fragmented any more, thus Path MTU discovery and ICMP is mandatory.
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