Any set of applications used to support e-mail messaging between a group of users in an organization or between users in different organizations. Mail systems include both client software (for composing, sending, receiving, and viewing messages) and server software (for storing and transporting messages).
Mail systems come in two general types:
Mail systems are also distinguished by their address formats. Each mail system uses one or more address formats. These can be industry-standard formats such as the Simple Mail Transfer Protocol (SMTP) or X.400 standard Originator/Recipient (O/R) format, or they can be proprietary address formats such as the Microsoft Mail version 3 or Lotus cc:Mail format. Exchange Server supports most popular industry-standard and proprietary address formats. The following table lists some of the address formats supported by Exchange.
| Address Format | Example |
| SMTP | mitch@northwind.microsoft.com |
| X.400 O/R | C=US;a=Sprint;p=microsoft;o=northwind;s=Tulloch;g=Mitch; |
| Microsoft Mail | microsoft/northwind/mitch |
| cc:Mail | Tulloch, Mitch at northwind |
Exchange Server can offer an all-in-one mail system solution because it can provide messaging connectivity with legacy mail systems such as Microsoft Mail and foreign mail systems such as Lotus cc:Mail, Internet SMTP mail, and X.400 messaging systems. This connectivity is provided by connectors such as the X.400 Connector and Internet Mail Service. You can thus use Exchange as a messaging backbone system in a heterogeneous mail system environment. Exchange Server also includes the Microsoft Exchange Migration Wizard, which you can use to migrate users and mailboxes from legacy and foreign mail systems to an Exchange-based system. This migration can simplify administration and reduce costs.