Problems with mail

Our network has a server running Linux Rad Hat 7.0 and Samba; and two gateways. On one gateway is a firewall, another is a mail gateway. We use a DSL line for Internet connection. PCs have Windows 2000 or Windows NT OS. Network worked well for a while, and then the problems started. In the morning, when I come, I cannot read my e-mail (our mail server is not available), although I can log in and retrieve data from server. When I reboot computers IN PARTICULAR ORDER (gw2, gw1 and finally server) after 5 minutes I can retrieve my mail. Any idea what is wrong? ONCE AGAIN, IT ALWAYS HAPPEN DURING THE NIGHT.