郵局協議（POP）是使用本地電子郵件客戶端可以檢索電子郵件從遠程服務器通過TCP/ IP連接的應用層網絡標準協議。 POP支持訪問遠程郵箱簡單的下載和 - 刪除要求。一個POP3服務器監聽熟知端口110。

軟件包com.sun.mail.pop3是POP3協議提供的JavaMail API，它提供了訪問的POP3郵件存儲。下表列出了類此包中：

名稱	描述
POP3Folder	A POP3 Folder (can only be "INBOX").
POP3Message	A POP3 Message.
POP3SSLStore	A POP3 Message Store using SSL.
POP3Store	A POP3 Message Store.

有幾點要注意此提供以上：

• POP3提供程序隻支持一個名為INBOX一個文件夾。由於POP3協議的限製，許多像事件通知，文件夾管理，標誌管理等JavaMail的API的功能是不允許的。

• POP3提供通過的JavaMail API的使用協議名稱POP3或以下形式的URL訪問 pop3://user:password@host:port/INBOX".

• POP3支持冇有永久的標誌。例如，Flags.Flag.RECENT標誌將永遠不會被用於POP3郵件設置。它是由應用程序，以確定哪些是在一個POP3郵箱的新郵件。

• POP3不支持Folder.expunge()方法。刪除並抹去消息，請將Flags.Flag.DELETED標誌上的信息，並使用Folder.close（true）方法關閉文件夾。

• POP3不提供接收日期，所以getReceivedDate方法將返回null。

• 當被訪問的POP3郵件的標題，在POP3提供程序使用top命令來獲取所有標題，然後將其緩存。

• 當訪問一個POP3郵件的內容時，POP3提供程序使用RETR命令來獲取整個消息。

• POP3Message.invalidate方法可用於緩存的無效數據，但不關閉文件夾。

POP3協議提供程序支持以下屬性，這些屬性可以在JavaMail會話對象進行設置。該屬性始終設置為字符串;類型列描述字符串是如何解釋的。

名稱	類型	描述
mail.pop3.user	String	Default user name for POP3.
mail.pop3.host	String	The POP3 server to connect to.
mail.pop3.port	int	The POP3 server port to connect to, if the connect() method doesn't explicitly specify one. Defaults to 110.
mail.pop3.connectiontimeout	int	Socket connection timeout value in milliseconds. Default is infinite timeout.
mail.pop3.timeout	int	Socket I/O timeout value in milliseconds. Default is infinite timeout.
mail.pop3.rsetbeforequit	boolean	Send a POP3 RSET command when closing the folder, before sending the QUIT command. Default is false.
mail.pop3.message.class	String	Class name of a subclass of com.sun.mail.pop3.POP3Message. The subclass can be used to handle (for example) non-standard Content-Type headers. The subclass must have a public constructor of the form MyPOP3Message(Folder f, int msgno) throws MessagingException.
mail.pop3.localaddress	String	Local address (host name) to bind to when creating the POP3 socket. Defaults to the address picked by the Socket class.
mail.pop3.localport	int	Local port number to bind to when creating the POP3 socket. Defaults to the port number picked by the Socket class.
mail.pop3.apop.enable	boolean	If set to true, use APOP instead of USER/PASS to login to the POP3 server, if the POP3 server supports APOP. APOP sends a digest of the password rather than the clear text password. Defaults to false.
mail.pop3.socketFactory	SocketFactory	If set to a class that implements the javax.net.SocketFactory interface, this class will be used to create POP3 sockets.
mail.pop3.socketFactory.class	String	If set, specifies the name of a class that implements the javax.net.SocketFactory interface. This class will be used to create POP3 sockets.
mail.pop3.socketFactory.fallback	boolean	If set to true, failure to create a socket using the specified socket factory class will cause the socket to be created using the java.net.Socket class. Defaults to true.
mail.pop3.socketFactory.port	int	Specifies the port to connect to when using the specified socket factory. If not set, the default port will be used.
mail.pop3.ssl.enable	boolean	If set to true, use SSL to connect and use the SSL port by default. Defaults to false for the "pop3" protocol and true for the "pop3s" protocol.
mail.pop3.ssl.checkserveridentity	boolean	If set to true, check the server identity as specified by RFC 2595. Defaults to false.
mail.pop3.ssl.trust	String	If set, and a socket factory hasn't been specified, enables use of a MailSSLSocketFactory.  If set to "*", all hosts are trusted.  If set to a whitespace separated list of hosts, those hosts are trusted.  Otherwise, trust depends on the certificate the server presents.
mail.pop3.ssl.socketFactory	SSLSocketFactory	If set to a class that extends the javax.net.ssl.SSLSocketFactory class, this class will be used to create POP3 SSL sockets.
mail.pop3.ssl.socketFactory.class	String	If set, specifies the name of a class that extends the javax.net.ssl.SSLSocketFactory class. This class will be used to create POP3 SSL sockets.
mail.pop3.ssl.socketFactory.port	int	Specifies the port to connect to when using the specified socket factory. If not set, the default port will be used.
mail.pop3.ssl.protocols	string	Specifies the SSL protocols that will be enabled for SSL connections. The property value is a whitespace separated list of tokens acceptable to the javax.net.ssl.SSLSocket.setEnabledProtocols method.
mail.pop3.starttls.enable	boolean	If true, enables the use of the STLS command (if supported by the server) to switch the connection to a TLS-protected connection before issuing any login commands. Defaults to false.
mail.pop3.starttls.required	boolean	If true, requires the use of the STLS command. If the server doesn't support the STLS command, or the command fails, the connect method will fail. Defaults to false.
mail.pop3.socks.host	string	Specifies the host name of a SOCKS5 proxy server that will be used for connections to the mail server.
mail.pop3.socks.port	string	Specifies the port number for the SOCKS5 proxy server.
mail.pop3.disabletop	boolean	If set to true, the POP3 TOP command will not be used to fetch message headers. Defaults to false.
mail.pop3.forgettopheaders	boolean	If set to true, the headers that might have been retrieved using the POP3 TOP command will be forgotten and replaced by headers retrieved as part of the POP3 RETR command. Defaults to false.
mail.pop3.filecache.enable	boolean	If set to true, the POP3 provider will cache message data in a temporary file rather than in memory. Messages are only added to the cache when accessing the message content. Message headers are always cached in memory (on demand). The file cache is removed when the folder is closed or the JVM terminates. Defaults to false.
mail.pop3.filecache.dir	String	If the file cache is enabled, this property can be used to override the default directory used by the JDK for temporary files.
mail.pop3.cachewriteto	boolean	Controls the behavior of the writeTo method on a POP3 message object. If set to true, and the message content hasn't yet been cached, and ignoreList is null, the message is cached before being written. Otherwise, the message is streamed directly to the output stream without being cached. Defaults to false.
mail.pop3.keepmessagecontent	boolean	If this property is set to true, a hard reference to the cached content will be kept, preventing the memory from being reused until the folder is closed or the cached content is explicitly invalidated (using the invalidate method). Defaults to false.

一般情況下，應用程序不應在此包中直接使用的類。相反，他們應該使用javax.mail的包定義的API（和子包）。應用程序不應該構建POP3Store或POP3Folder直接的實例。相反，他們應該使用Session方法getStore獲得適當的存儲對象，並從獲取文件夾對象。

示例使用的POP3服務器是體現在 檢查電子郵件.