Question

I’m working on an IMAP client using Ruby and Rails. I can successfully import messages, mailboxes, and more… However, after the initial import, how can I detect any changes that have occurred since my last sync?

Currently I am storing the UIDs and UID validity values in the database, comparing them, and searching appropriately. This works, but it doesn’t detect deleted messages or changes to message flags, etc.

Do I have to pull all messages every time to detect these changes? How do other IMAP clients do it so quickly (i.e. Apple Mail and Postbox). My script is already taking 10+ seconds per account with very few email addresses:

# select ourself as the current mailbox
@imap_connection.examine(self.location)

# grab all new messages and update them in the database
# if the uid's are still valid, we will just fetch the newest UIDs
# otherwise, we need to search when we last synced, which is slower :(
if self.uid_validity.nil? || uid_validity == self.uid_validity
  # for some IMAP servers, if a mailbox is empty, a uid_fetch will fail, so then
  begin
    messages = @imap_connection.uid_fetch(uid_range, ['UID', 'RFC822', 'FLAGS'])
  rescue
    # gmail cries if the folder is empty
    uids = @imap_connection.uid_search(['ALL'])
    messages = @imap_connection.uid_fetch(uids, ['UID', 'RFC822', 'FLAGS']) unless uids.empty?
  end

  messages.each do |imap_message|
    Message.create_from_imap!(imap_message, self.id)
  end unless messages.nil?
else
  query = self.last_synced.nil? ? ['All'] : ['SINCE', Net::IMAP.format_datetime(self.last_synced)]
  @imap_connection.search(query).each do |message_id|
    imap_message = @imap_connection.fetch(message_id, ['RFC822', 'FLAGS', 'UID'])[0]

    # don't mark the messages as read
    #@imap_connection.store(message_id, '-FLAGS', [:Seen])

    Message.create_from_imap!(imap_message, self.id)
  end
end

# now assume all UIDs are valid
self.uid_validity = uid_validity

# now remember that we just fetched all those messages
self.last_synced = Time.now
self.save!

Answer 1

There is an IMAP extension for Quick Flag Changes Resynchronization (RFC-4551). With this extension it is possible to search for all messages that have been changed since the last synchronization (based on some kind of timestamp). However, as far as I know this extension is not widely supported.

There is an informational RFC that describes how IMAP clients should do synchronization (RFC-4549, section 4.3). The text recommends issuing the following two commands:

tag1 UID FETCH <lastseenuid+1>:* <descriptors>
tag2 UID FETCH 1:<lastseenuid> FLAGS

The first command is used to fetch the required information for all unknown mails (without knowing how many mails there are). The second command is used to synchronize the flags for the already seen mails.

AFAIK this method is widely used. Therefore, many IMAP servers contain optimizations in order to provide this information quickly. Typically, the network bandwidth is the limiting factor.