Relay Servers

In some situations it may not be possible to send mail directly to the server that accepts mail for a given domain. The two most common situations are corporate networks which have centralized servers that are responsible for delivering and forwarding messages, or an Internet Service Provider (ISP) which specifically blocks access to all mail servers other than their own. This is usually done as either a security measure or as a means to inhibit users from sending unsolicited commercial email messages. If the standard SMTP port is being blocked, then any connection attempts will either fail immediately with an error that the server is unreachable, or the connection will simply time-out. In either case, a relay server must be specified in order to send email messages.

A relay server is a system which will accept messages addressed to users which may be in a different domain, and will relay those messages to the appropriate server that does accept mail for the domain. In that case, the HostName property will specify the host name or address of the mail server that will be used by the control to relay the message. In some cases, the relay server may use a non-standard port number to circumvent blocks on port 25. In that case, the RemotePort property should be set to the port number used by the relay server.

It is important to note that using a mail server as a relay without the permission of the organization or individual who owns that server may violate Acceptable Use Policies and/or Terms of Service agreements with your service provider. Systems which relay messages from anyone, regardless of whether the message is coming from a recognized domain, are called open relays. Because open relays are often used to send unsolicited email, many administrators block mail that comes from one. It is recommended that users check with their network administrators or Internet service providers to determine if access to external mail servers is restricted and what is the acceptable use policy for relaying messages through their mail servers.