External IP communication

I am in the unenviable position of having been dumped with our network administration! Our internal network is fine, but some users require FTP services from outside our network and cannot access these sites. They say they have been able to connect last week.

We have a random set of internal IP addresses connected to our ISP via our Mailserver (with Proxy Server etc.) having a second card with a valid Internet address.

As far as I know, nothing has been changed on the server from the timeit was working until now???

Thanks for any ideas.