User guide

Chapter 4. Your Internet Service Provider (ISP)
Secondary Mail Server
A secondary mail server receives e-mail for your domain if your server is unavailable, and reattempts delivery later.
Subnet Mask (or netmask)
A subnet mask (or a netmask) has four numeric segments (each between 0 and 255) and looks like an IP address. It enables your computers to
deduce what network they are on. Your ISP provides the netmask for the external network between the ISP and your server.
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