In addition to basic message translation functions, a gateway might also have to provide more advanced functions including translating document formats, routing according to cost of route, notification of nondelivery and, lastly, to ensure that the directories of the two mail systems are synchronized.

Unfortunately, there are problems with gateways. They can often lose information, especially embedded objects, multimedia or rich text formatting. Sometimes, addresses are not correctly filled out by the sender and the gateway cannot deliver the message. In addition, some functions, especially Receipt don't work across gateways. Applications that use API calls to provide mail-enabled functions rarely work across gateways. However, in spite of all these problems, if you have two mail systems, a gateway is still the simplest and most effective way of connecting them. Added to this is the benefit that almost every e-mail package has a wide variety of gateways available.

If your e-mail package does not have a gateway for the second mail system that want to integrate you will have to turn to a middle standard which is supported by both packages. There are three main independent standards: X.400, SMTP, and MHS all three are discussed later.