Backup MX port

[Robert Sharp]

Hi,

I have been trying to use a third-party backup provider to cover 
occasional outages (SiXwishlist). I set up an MX record as instructed 
but I get no emails sent on when my server recovers. I asked them to 
check what was happening and they told me my port was not open. It seems 
they were expecting postfix to be listening to port 4000. I asked for 
clarification and got a response that they would send emails on 587, 
2525, 3000, and 4000, but that 25 was not recommended and "is blocked 
for 90% of service hosting providers".

I thought that smtp/25 was the backbone of email systems. I use 587 
internally for submission so I don't see why I would make it accessible 
to the outside world. Do I need to open another port? Have I got port 25 
wrong all this time? Or do I need to get myself a proper backup provider?

Thanks,
Robert Sharp