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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
Initialize the control and load the Windows Sockets library 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.
Connect
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
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client session is no longer valid.
Uninitialize
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.
Message
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.
MessageSize
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.
GetMessage
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.
StoreMessage
This method downloads a complete message and stores it as a text
file on the local system.
DeleteMessage
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:
GetHeaders
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.
GetHeader
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.
MessageUID
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.
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