The Internet Mail .NET component is designed to minimize the differences between the Post Office Protocol (POP3) and Internet Message Access Protocol (IMAP4), as well as present a consistent and uniform interface for parsing and sending e-mail messages. In general, the type of mail server that you use will be determined by the port number or URL. When retrieving messages from a mail server, you would typically perform the following actions:

• Establish a connection to the mail server using the Connect method.
• If connected to an IMAP4 server, optionally select a mailbox using the SelectMailbox method.
• Use the MessageCount property to determine the number of messages available.
• Use the GetMessage to retrieve one or more messages from the server.
• Terminate the connection to the server using the Disconnect method.

When submitting a message for delivery to one or more recipients, you would typically perform the following actions:

• Create the message using the ComposeMessage method.
• Optionally attach one or more files using the AttachFile method.
• Submit the message to the mail server for delivery using the SendMessage method.

When using the Internet Mail .NET component to send e-mail messages, it is important to keep in mind that most system administrators have security measures and restrictions designed to prevent the delivery of unsolicited messages (spam) through their mail servers. For example, they may require that you authenticate with a username and password before being permitted to submit a message for delivery. Other systems may block access to your application if you are connecting from a dial-up networking account. Many Internet service providers (ISPs) also block access to the standard port for the Simple Mail Transfer Protocol except for their own mail servers, forcing you to route all outbound messages through their mail servers so they can monitor activity for abuse. For more information about the specific restrictions imposed by your mail service provider, we recommend that you contact their system administrator or technical support staff.

Service Ports

By convention, most mail servers accept connections on the following ports:

If you leave the ServerPort property at its default value of zero, the correct port number will be automatically selected according to the value of the ServerType and Secure properties. However, you can override the standard port values by setting the ServerPort property. This enables your application to establish a connection with a server that is configured to use non-standard port numbers.

Authentication

Mail servers using the Post Office Protocol and Internet Message Access Protocol both require that clients authenticate with a username and password. When sending e-mail messages, the mail server may also require authentication if you are attempting to relay a message through the server. This means that if you are attempting to send a message that requires the mail server to forward the message on to the recipient (because the message is not addressed to a local user), then the mail server will typically require that you authenticate the session. Those mail servers which do not require authentication are known as "open relays" and are typically blocked by other mail servers because they are commonly used to send unsolicited e-mail messages.

Protocol Standards

There are five core standards which form the foundation for sending and receiving e-mail messages over the Internet and corporate intranets. These standards are defined in documents called RFCs (Request For Comments) which describe how the various protocols should be implemented. The following standards were used when implementing the Internet Mail control:

RFC 822 documents the basic structure of e-mail messages, including how messages should be formatted and what the standard message header fields are. RFC 2045 documents Multipurpose Internet Mail Extensions (MIME), which details how more complicated messages are structured. File attachments, HTML formatted messages and other more complex aspects of message composition are covered by the MIME standard. The Internet Mail control supports both RFC 822 and MIME formatted e-mail messages, including multipart messages which contain alternate text and file attachments.

RFC 1939 documents the Post Office Protocol (POP3) which is used to retrieve messages from a user's mailbox on a server. The Internet Mail control uses this protocol to enable applications to list, retrieve and delete messages.

RFC 821 documents the Simple Mail Transfer Protocol (SMTP) which is used to deliver messages to one or more recipients. RFC 1869 documents extensions to the protocol which provide additional services such as delivery status notification and authentication. The Internet Mail control implements both the standard and extended SMTP protocols.

---