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The Post Office Protocol (POP3) library enables an application to
retrieve a user's mail messages and store them on the local system. The
POP3 API 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 library also has functions for changing the user's
password and sending messages if they are supported by the server.
The first step your application must take is to initialize the
library, then establish a connection to the server and authenticate the
client. The following functions are available for use by your
application:
PopInitialize
Initialize the library and load the Windows Sockets library for the
current process. This must be the first function call the application
makes before calling the other POP3 API functions.
PopConnect
Establish a connection to the POP3 server. This function will return
a handle to a client session which is used in subsequent calls to the
POP3 API.
PopLogin
Authenticate yourself to the server using a username and password.
This function should be called immediately after the connection has
been established to the server. You can specify either the standard
authentication method, or the APOP authentication method if required
by the server.
PopDisconnect
Disconnect from the POP3 server and release any resources that have
been allocated for the client session. After this function is called,
the client handle is no longer valid.
PopUninitialize
Unload the Windows Sockets library and release any resources that
have been allocated for the current process. This is the last
function call the application should make prior to terminating.
Managing Messages
There are functions in the POP3 library 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.
PopGetMessageCount
Return 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.
PopGetMessageCountEx
An extended version of the PopGetMessageCount function, this function
will return the number of available messages, along with the last
available message number and the total size of all messages in the
mailbox.
PopGetMessage
Retrieve a message from the server, storing the contents in memory.
This can be used to process the contents of a message without the
overhead of storing it in a file on the local system.
PopStoreMessage
This function downloads a complete message and stores it as a text
file on the local system.
PopDeleteMessage
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 PopReset function instead
of calling PopDisconnect.
PopGetMessageSize
This function returns the size of the message in bytes. One thing to
be aware of when using this function 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.
Message Headers
The POP3 API also includes functions 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 functions can be used to examine the
headers in a message:
PopGetMessageHeaders
This function 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.
PopGetHeaderValue
This function returns the value for a specific header field in a
message. This function 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 function fails with the
error that the feature is not supported, you should use the
PopGetMessageHeaders function instead.
PopGetMessageId
This function returns the value of the Message-ID header in the
specified message. This is a unique string that is used to identify
the message. Note that it is not the same as the UID value returned
by the POP3 server.
PopGetMessageUid
This function 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.
PopGetMessageSender
This function returns the email address of the person who sent the
message. This function requires that the server support extended POP3
commands. If the server does not support the command used to retrieve
the sender, it will return a value of zero. Applications should not
depend on this function returning a valid address. Typically it is
used for informational purposes, such as displaying the sender to the
user as a message is being retrieved.
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