Once again may I suggest having a look at OpenVPN.

Once you have a tunnel through to your mailserver on any port you choose so it should not get blocked by your ISP.

You will have access to the local IP address of the mail server (the other end of the tunnel).

you can then use

mailserver.vpntunnel.ip.address:25

from the N900

All you need then is to tell the mailserver to allow relay from the vpn tunnel address range.

rgds