Post Office Protocol  

The Post Office Protocol (POP3) control enables an application to retrieve a user's mail messages and store them on the local system. The control provides support for all of the standard functionality such as listing and downloading messages, as well as extended features such as the ability to retrieve only the headers for a message or just specific header values. The control also has methods for changing the user's password and sending messages if they are supported by the server.

The following properties, methods and events are available for use by your application:

Initialize the control and validate the runtime license key for the current process. This method is normally not used if the control is placed on a form in languages such as Visual Basic. However, if the control is being created dynamically using a method similar to CreateObject, then the application must call this method to initialize the component before setting any properties or calling any other methods in the control.

Establish a connection to the POP3 server. Once the connection has been established, the other methods in the control may be used to access the resources on the server.

Disconnect from the server and release any resources that have been allocated for the client session. After this method is called, the client session is no longer valid.

Unload the Windows Sockets library and release any resources that have been allocated for the current process. This is the last method call that the application should make prior to terminating. This is only necessary if the application has previously called the Initialize method.

Managing Messages

There are methods in the POP3 control for managing messages which enables the application to list, delete and retrieve messages stored on the server. Messages are identified by a number, starting with one for the first message in the mailbox. The most typical operation for a POP3 client is to retrieve each message, store it on the local system and then delete the message from the server. Any processing that is done on the message would then be done on the local copy.

This property sets or returns the message number for the currently selected mailbox. Message numbers range from 1 through the number of messages available on the server, as returned by the MessageCount property.

MessageCount, LastMessage
A property which returns the number of messages available for retrieval. There are two values the application should use. One is the number of currently available messages and the other is the last valid message number. As messages are deleted from the server, the total number of available messages will decrease; however, the last available message number will remain constant.

This property returns the size of the message in bytes. One thing to be aware of when using this method is that some servers will only return approximate message sizes. In addition, because of the difference between the end-of-line characters on UNIX and Windows systems, the size reported by the server may not be the actual size of the message when stored on the local system. Therefore, the application should not depend on this value as an absolute. For example, it should not use this value to determine the maximum number of bytes to read from the server; instead, it should read until the server indicates that the end of the message has been reached.

This method is used to retrieve a message from the server and copy it into a local string or byte array buffer. This method will cause the current thread to block until the message transfer completes, a timeout occurs or the transfer is canceled.

This method downloads a complete message and stores it as a text file on the local system.

Mark the message for deletion. When the connection with the server is closed, the message will be removed from the user's inbox. An important difference between the POP3 and IMAP protocols is that when a message is marked as deleted on a POP3 server, that message can no longer be accessed. An attempt to retrieve a message after it has been marked for deletion will result in an error. The only way to undelete a message once it has been deleted is to terminate the connection with the server by calling the Reset method instead of calling the Disconnect method.

Message Headers

The POP3 control also includes methods which enable the application to access just the headers for a message. This can be useful if the program doesn't want to incur the overhead of downloading the entire message contents. The following methods can be used to examine the headers in a message:

This method returns the complete set of headers for the specified message. If your program has to process multiple header fields, this is the most efficient method to use. It is possible to retrieve specific header values, however not all servers support that option and it is somewhat slower because it involves sending individual commands to request each value.

This method returns the value for a specific header field in a message. This method does not require that you parse the message headers; however it does incur additional overhead. It is also important to note that not all servers support the command that is used to request the header value. If this method fails with the error that the feature is not supported, you should use the GetHeaders method instead.

This property returns the unique ID (UID) that the server has associated with the message.  The UID can be used by an application to track whether or not it has previously viewed the message. Unlike the message number, which can change between client sessions, the message UID is guaranteed to be the same value across sessions until the message is deleted.