problem with FTP Security Server

[Nexus6]

Hello,
I have a cluster of firewalls running CheckPoint VPN-1/FW-1 R55 HFA_02 on ipso 3.8 BUILD 034 (Nokia)

I'm having a problem with the behaviuor of the FTP security server.
From client side, to initiate an ftp session the customers usually do like this :

ftp x.x.x.x
login : remote_user_login@proxy_user_login@remote_ftp_site
password : remote_password@proxy_user_password

and everything works.

The problem arises when the remote password must be an e-mail account, so it contains itself the @ character. At this point the FTP security server goes confused and the doesn't connect to the remote FTP site.

Does anyone knows how to solve this problem ?