Passive Mode Failing Through a SonicWALL - KB Article #1881
Related Articles --Sometimes connecting to Serv-U through a SonicWALL using PASV mode has been known to fail. This can sometimes be due to the Serv-U Listener having a specified PASV IP Address when the SonicWALL is NAT (Network Address Translation) enabled. You may be able to get around this issue by not specifying a specific IP address for the "PASV IP Address or Domain Name" field in the Serv-U listener and leaving this field blank.