Programming Tutorials

IMAP in Ruby

By: James Edward Gray II in Ruby Tutorials on 2009-03-03  

Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality. The protocol is described in [IMAP]. 

An IMAP client connects to a server, and then authenticates itself using either authenticate() or login(). Having authenticated itself, there is a range of commands available to it. Most work with mailboxes, which may be arranged in an hierarchical namespace, and each of which contains zero or more messages. How this is implemented on the server is implementation-dependent; on a UNIX server, it will frequently be implemented as a files in mailbox format within a hierarchy of directories. 

To work on the messages within a mailbox, the client must first select that mailbox, using either select() or (for read-only access) examine(). Once the client has successfully selected a mailbox, they enter selected state, and that mailbox becomes the current mailbox, on which mail-item related commands implicitly operate. 

Messages have two sorts of identifiers: message sequence numbers, and UIDs. 

Message sequence numbers number messages within a mail box from 1 up to the number of items in the mail box. If new message arrives during a session, it receives a sequence number equal to the new size of the mail box. If messages are expunged from the mailbox, remaining messages have their sequence numbers "shuffled down" to fill the gaps. 

UIDs, on the other hand, are permanently guaranteed not to identify another message within the same mailbox, even if the existing message is deleted. UIDs are required to be assigned in ascending (but not necessarily sequential) order within a mailbox; this means that if a non-IMAP client rearranges the order of mailitems within a mailbox, the UIDs have to be reassigned. An IMAP client cannot thus rearrange message orders. 

Examples of Usage
List sender and subject of all recent messages in the default mailbox

  imap = Net::IMAP.new('mail.example.com')
  imap.authenticate('LOGIN', 'joe_user', 'joes_password')
  imap.examine('INBOX')
  imap.search(["RECENT"]).each do |message_id|
    envelope = imap.fetch(message_id, "ENVELOPE")[0].attr["ENVELOPE"]
    puts "#{envelope.from[0].name}: \t#{envelope.subject}"
  end

Move all messages from April 2003 from "Mail/sent-mail" to "Mail/sent-apr03"

  imap = Net::IMAP.new('mail.example.com')
  imap.authenticate('LOGIN', 'joe_user', 'joes_password')
  imap.select('Mail/sent-mail')
  if not imap.list('Mail/', 'sent-apr03')
    imap.create('Mail/sent-apr03')
  end

  imap.search(["BEFORE", "30-Apr-2003", "SINCE", "1-Apr-2003"]).each do |message_id|
    imap.copy(message_id, "Mail/sent-apr03")
    imap.store(message_id, "+FLAGS", [:Deleted])
  end
  imap.expunge

Thread Safety
Net::IMAP supports concurrent threads. For example, 

  imap = Net::IMAP.new("imap.foo.net", "imap2")
  imap.authenticate("cram-md5", "bar", "password")
  imap.select("inbox")
  fetch_thread = Thread.start { imap.fetch(1..-1, "UID") }
  search_result = imap.search(["BODY", "hello"])
  fetch_result = fetch_thread.value
  imap.disconnect

Errors

An IMAP server can send three different types of responses to indicate failure: 

NO: the attempted command could not be successfully completed. For instance, the username/password used for logging in are incorrect; the selected mailbox does not exists; etc. 
BAD: the request from the client does not follow the server's understanding of the IMAP protocol. This includes attempting commands from the wrong client state; for instance, attempting to perform a SEARCH command without having SELECTed a current mailbox. It can also signal an internal server failure (such as a disk crash) has occurred. 
BYE: the server is saying goodbye. This can be part of a normal logout sequence, and can be used as part of a login sequence to indicate that the server is (for some reason) unwilling to accept our connection. As a response to any other command, it indicates either that the server is shutting down, or that the server is timing out the client connection due to inactivity. 

These three error response are represented by the errors Net::IMAP::NoResponseError, Net::IMAP::BadResponseError, and Net::IMAP::ByeResponseError, all of which are subclasses of Net::IMAP::ResponseError. Essentially, all methods that involve sending a request to the server can generate one of these errors. Only the most pertinent instances have been documented below. 

Because the IMAP class uses Sockets for communication, its methods are also susceptible to the various errors that can occur when working with sockets. These are generally represented as Errno errors. For instance, any method that involves sending a request to the server and/or receiving a response from it could raise an Errno::EPIPE error if the network connection unexpectedly goes down. See the socket(7), ip(7), tcp(7), socket(2), connect(2), and associated man pages. 

Finally, a Net::IMAP::DataFormatError is thrown if low-level data is found to be in an incorrect format (for instance, when converting between UTF-8 and UTF-16), and Net::IMAP::ResponseParseError is thrown if a server response is non-parseable.