In the figure, the MDA accepts a piece of email from a MTA and performs the actual delivery. The MDA receives all the inbound mail from the MTA and places it into the appropriate users’ mailboxes. The MDA can also resolve final delivery issues, such as virus scanning, spam filtering, and return-receipt handling. Most email communications use the MUA, MTA, and MDA applications; however, there are other alternatives for email delivery.
A client may be connected to a corporate email system, such as IBM’s Lotus Notes, Novell’s Groupwise, or Microsoft’s Exchange. These systems often have their own internal email format, and their clients typically communicate with the email server using a proprietary protocol. The server sends or receives email via the Internet through the product’s Internet mail gateway, which performs any necessary reformatting.
As another alternative, computers that do not have an email client can still connect to a mail service on a web browser to retrieve and send messages. Some computers may run their own MTA and manage interdomain email. If, for example, two people who work for the same company exchange email with each other using a proprietary protocol, their messages may stay completely within the company’s corporate email system.