When running Redmine::IMAP.check(imap_options, options) in a continuous loop with a delay (I don't want to boot the rails env every time) you will eventually get the error listed below depending on your mail provider's maximum concurrent users:

rake aborted!
 Too many simultaneous connections. (Failure)
/usr/local/lib/ruby/2.0.0/net/imap.rb:1156:in `get_tagged_response'
/usr/local/lib/ruby/2.0.0/net/imap.rb:1210:in `block in send_command'
/usr/local/lib/ruby/2.0.0/monitor.rb:211:in `mon_synchronize'
/usr/local/lib/ruby/2.0.0/net/imap.rb:1192:in `send_command'
/usr/local/lib/ruby/2.0.0/net/imap.rb:435:in `login'
/home/ppretorius/workspace-ruby/redmine-2.3.1/lib/redmine/imap.rb:30:in `check'

This problem is fixed by simply adding imap.logout and imap.disconnect at the end of the method:

  imap.expunge
  imap.logout
  imap.disconnect

I got the solution "from this stackoverflow post":http://stackoverflow.com/questions/3322510/ruby-netimap-closing-a-connection.