A number of people uses more than one SMTP server. An SMTP server is the computer program, managed by your internet provider (employer, school, ISP, ...), that takes care of sending your email. Such an SMTP server can be a bare programs marginally implementing the standard RFC protocols, or it can be part of a larger server program, like the Microsoft Exchange Server.
More than one SMTP Server
There are good reasons for a user wanting to have more than one SMTP server available. Network servers go down occasionally. The time and the frequency they faulter varies, but faulter they do. When your SMTP server is unavailable you cannot send your mission-critical email. As a safety net having a few extra SMTP servers at your disposal comes in quite handy.
Protection of SMTP servers
Another reason for having more freedom in picking out different SMTP server is the way they are protected. An SMTP server must be protected against "relaying" all email-send requests. If this protection would be absent the SMTP sever would immediately be used for relaying spam. There are basically three SMTP protections: >> >>