A POP3 protocol provider for the JavaMail API that provides access to a POP3 message store. Refer to RFC 1939 for more information.

The POP3 provider provides a Store object that contains a single Folder named "INBOX". Due to the limitations of the POP3 protocol, many of the JavaMail API capabilities like event notification, folder management, flag management, etc. are not allowed. The corresponding methods throw the MethodNotSupportedException exception; see below for details.

Note that JavaMail does not include a local store into which messages can be downloaded and stored. See our Third Party Products web page for availability of "mbox" and "MH" local store providers.

The POP3 provider is accessed through the JavaMail APIs by using the protocol name "pop3" or a URL of the form "pop3://user:password@host:port/INBOX".

POP3 supports only a single folder named "INBOX".

POP3 supports no permanent flags (see {@link javax.mail.Folder#getPermanentFlags Folder.getPermanentFlags()}). In particular, the `Flags.Flag.RECENT` flag will never be set for POP3 messages. It's up to the application to determine which messages in a POP3 mailbox are "new". There are several strategies to accomplish this, depending on the needs of the application and the environment:

- A simple approach would be to keep track of the newest message seen by the application.
- An alternative would be to keep track of the UIDs (see below) of all messages that have been seen.
- Another approach is to download all messages into a local mailbox, so that all messages in the POP3 mailbox are, by definition, new.

All approaches will require some permanent storage associated with the client.

POP3 does not support the `Folder.expunge()` method. To delete and expunge messages, set the `Flags.Flag.DELETED` flag on the messages and close the folder using the `Folder.close(true)` method. You cannot expunge without closing the folder.

POP3 does not provide a "received date", so the `getReceivedDate` method will return null. It may be possible to examine other message headers (e.g., the "Received" headers) to estimate the received date, but these techniques are error-prone at best.

The POP3 provider supports the POP3 UIDL command, see {@link com.sun.mail.pop3.POP3Folder#getUID POP3Folder.getUID()}. You can use it as follows:

>     if (folder instanceof com.sun.mail.pop3.POP3Folder) {
>         com.sun.mail.pop3.POP3Folder pf =
>     	(com.sun.mail.pop3.POP3Folder)folder;
>         String uid = pf.getUID(msg);
>         if (uid != null)
>     	... // use it
>     }

You can also pre-fetch all the UIDs for all messages like this:

>     FetchProfile fp = new FetchProfile();
>     fp.add(UIDFolder.FetchProfileItem.UID);
>     folder.fetch(folder.getMessages(), fp);

Then use the technique above to get the UID for each message. This is similar to the technique used with the UIDFolder interface supported by IMAP, but note that POP3 UIDs are strings, not integers like IMAP UIDs. See the POP3 spec for details.

When the headers of a POP3 message are accessed, the POP3 provider uses the TOP command to fetch all headers, which are then cached. Use of the TOP command can be disabled with the `mail.pop3.disabletop` property, in which case the entire message content is fetched with the RETR command.

When the content of a POP3 message is accessed, the POP3 provider uses the RETR command to fetch the entire message. Normally the message content is cached in memory. By setting the `mail.pop3.filecache.enable` property, the message content will instead be cached in a temporary file. The file will be removed when the folder is closed. Caching message content in a file is generally slower, but uses substantially less memory and may be helpful when dealing with very large messages.

The {@link com.sun.mail.pop3.POP3Message#invalidate POP3Message.invalidate} method can be used to invalidate cached data without closing the folder. Note that if the file cache is being used the data in the file will be forgotten and fetched from the server if it's needed again, and stored again in the file cache.

The POP3 CAPA command (defined by RFC 2449) will be used to determine the capabilities supported by the server. Some servers don't implement the CAPA command, and some servers don't return correct information, so various properties are available to disable use of certain POP3 commands, including CAPA.

If the server advertises the PIPELINING capability (defined by RFC 2449), or the `mail.pop3.pipelining` property is set, the POP3 provider will send some commands in batches, which can significantly improve performance and memory use. Some servers that don't support the CAPA command or don't advertise PIPELINING may still support pipelining; experimentation may be required.

If pipelining is supported and the connection is using SSL, the USER and PASS commands will be sent as a batch. (If SSL is not being used, the PASS command isn't sent until the user is verified to avoid exposing the password if the user name is bad.)

If pipelining is supported, when fetching a message with the RETR command, the LIST command will be sent as well, and the result will be used to size the I/O buffer, greatly reducing memory usage when fetching messages.

#### Properties

The POP3 protocol provider supports the following properties, which may be set in the JavaMail `Session` object. The properties are always set as strings; the Type column describes how the string is interpreted. For example, use

    	props.put("mail.pop3.port", "888");

to set the `mail.pop3.port` property, which is of type int.

Note that if you're using the "pop3s" protocol to access POP3 over SSL, all the properties would be named "mail.pop3s.*".

Name | Type | Description
--- | --- | ---
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. This timeout is implemented by java.net.Socket. Default is infinite timeout.
mail.pop3.timeout | int | Socket read timeout value in milliseconds. This timeout is implemented by java.net.Socket. Default is infinite timeout.
mail.pop3.writetimeout | int | Socket write timeout value in milliseconds. This timeout is implemented by using a java.util.concurrent.ScheduledExecutorService per connection that schedules a thread to close the socket if the timeout expires. Thus, the overhead of using this timeout is one thread per connection. Default is infinite timeout.
mail.pop3.rsetbeforequit | boolean | Send a POP3 RSET command when closing the folder, before sending the QUIT command. Useful with POP3 servers that implicitly mark all messages that are read as "deleted"; this will prevent such messages from being deleted and expunged unless the client requests so. 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. Should not normally need to be set, but useful with multi-homed hosts where it's important to pick a particular local address to bind to.
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. Note that this is an instance of a class, not a name, and must be set using the `put` method, not the `setProperty` method.
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. These additional checks based on the content of the server's certificate are intended to prevent man-in-the-middle attacks. Defaults to false.
mail.pop3.ssl.trust | String | If set, and a socket factory hasn't been specified, enables use of a {@link com.sun.mail.util.MailSSLSocketFactory 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. Note that this is an instance of a class, not a name, and must be set using the `put` method, not the `setProperty` method.
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.ssl.ciphersuites | string | Specifies the SSL cipher suites that will be enabled for SSL connections. The property value is a whitespace separated list of tokens acceptable to the `javax.net.ssl.SSLSocket.setEnabledCipherSuites` 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. Note that an appropriate trust store must configured so that the client will trust the server's certificate. 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. (Note that this only works on JDK 1.5 or newer.)
mail.pop3.socks.port | string | Specifies the port number for the SOCKS5 proxy server. This should only need to be used if the proxy server is not using the standard port number of 1080.
mail.pop3.disabletop | boolean | If set to true, the POP3 TOP command will not be used to fetch message headers. This is useful for POP3 servers that don't properly implement the TOP command, or that provide incorrect information in the TOP command results. Defaults to false.
mail.pop3.disablecapa | boolean | If set to true, the POP3 CAPA command will not be used to fetch server capabilities. This is useful for POP3 servers that don't properly implement the CAPA command, or that provide incorrect information in the CAPA command results. 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. Some servers, such as some versions of Microsft Exchange and IBM Lotus Notes, will return slightly different headers each time the TOP or RETR command is used. To allow the POP3 provider to properly parse the message content returned from the RETR command, the headers also returned by the RETR command must be used. Setting this property to true will cause these headers to be used, even if they differ from the headers returned previously as a result of using the TOP 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 {@link com.sun.mail.pop3.POP3Message#writeTo 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 | The content of a message is cached when it is first fetched. Normally this cache uses a {@link java.lang.ref.SoftReference SoftReference} to refer to the cached content. This allows the cached content to be purged if memory is low, in which case the content will be fetched again if it's needed. 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 {@link com.sun.mail.pop3.POP3Message#invalidate invalidate} method). (This was the behavior in previous versions of JavaMail.) Defaults to false.
mail.pop3.finalizecleanclose | boolean | When the finalizer for POP3Store or POP3Folder is called, should the connection to the server be closed cleanly, as if the application called the close method? Or should the connection to the server be closed without sending any commands to the server? Defaults to false, the connection is not closed cleanly.

In general, applications should not need to use the classes in this package directly. Instead, they should use the APIs defined by `javax.mail` package (and subpackages). Applications should never construct instances of `POP3Store` or `POP3Folder` directly. Instead, they should use the `Session` method `getStore` to acquire an appropriate `Store` object, and from that acquire `Folder` objects.

In addition to printing debugging output as controlled by the {@link javax.mail.Session Session} configuration, the com.sun.mail.pop3 provider logs the same information using {@link java.util.logging.Logger} as described in the following table:

Logger Name | Logging Level | Purpose
--- | --- | ---
com.sun.mail.pop3 | CONFIG | Configuration of the POP3Store
com.sun.mail.pop3 | FINE | General debugging output
com.sun.mail.pop3.protocol | FINEST | Complete protocol trace

WARNING: The APIs unique to this package should be considered EXPERIMENTAL. They may be changed in the future in ways that are incompatible with applications using the current APIs.