Rails app won't send mail (via sendmail) under JRuby
class MessageMailer < ActionMailer::Base
  def message(msg, recipient, reply_to_email=nil)
    template = (msg.message_type.nil?) ? "default" : msg.message_type.name.downcase.gsub(' ', '_')

    recipients recipient
    subject msg.subject
    from (msg.sender.nil? or msg.sender.email.blank?) ? ""no-reply" <#{SYSTEM_EMAIL_ADDRESS}>" : msg.sender.email
    content_type "text/html"
    body render_message(template, :message => msg)
    reply_to reply_to_email || ((msg.sender.nil? or msg.sender.email.blank?) ? ""no-reply" <#{SYSTEM_EMAIL_ADDRESS}>" : msg.sender.email)
  end

  ...
end

MessageMailer.deliver_message(...)

config.action_mailer.delivery_method = :sendmail

# Sending mail under MRI Ruby 1.8.7
Jan  5 09:38:49 my sendmail[24755]: q05EcnCr024755: from=edwarda, size=310, class=0, nrcpts=1, msgid=<201201051438.q05EcnCr024755@my.example.org>, relay=edwarda@localhost
Jan  5 09:38:49 my sm-mta[24757]: q05Ecn02024757: from=<edwarda@my.example.org>, size=516, class=0, nrcpts=1, msgid=<201201051438.q05EcnCr024755@my.example.org>, proto=ESMTP, daemon=MTA-v4, relay=localhost [127.0.0.1]
Jan  5 09:38:49 my sendmail[24755]: q05EcnCr024755: to=me@example.com, ctladdr=edwarda (1011/1012), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30310, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (q05Ecn02024757 Message accepted for delivery)
Jan  5 09:38:49 my sm-mta[24759]: STARTTLS=client, relay=aspmx.l.google.com., version=TLSv1/SSLv3, verify=FAIL, cipher=RC4-SHA, bits=128/128
Jan  5 09:38:49 my sm-mta[24759]: q05Ecn02024757: to=<me@example.com>, ctladdr=<edwarda@my.example.org> (1011/1012), delay=00:00:00, xdelay=00:00:00, mailer=esmtp, pri=120516, relay=aspmx.l.google.com. [74.125.45.27], dsn=2.0.0, stat=Sent (OK 1325774329 o43si18661797yhk.140)

# Sending mail under JRuby 1.6.5
Jan  5 11:10:26 my sendmail[7623]: q05GAQkH007623: from=edwarda, size=199, class=0, nrcpts=0, relay=edwarda@localhost

gem 'activerecord-jdbc-adapter', '<= 1.2.0', :require => false
gem 'activerecord-jdbcpostgresql-adapter', '<= 1.2.0', :require => 'jdbc_adapter'
gem 'ffi-ncurses'
gem 'jruby-openssl'
gem 'torquebox', '2.0.0.beta1', :platforms => 'jruby'
gem 'torquebox-rake-support', :platforms => 'jruby'
gem 'torquebox-capistrano-support', '2.0.0.beta1'

module ActionMailer
  class Base
    def perform_delivery_sendmail(mail)
      sendmail_args = sendmail_settings[:arguments]
      sendmail_args += " -f "#{mail['return-path']}"" if mail['return-path']

      IO.popen("#{sendmail_settings[:location]} #{sendmail_args}", "r+") do |f| #r+ geht, w+ geht nicht richtig, bzw. nur manchmal
        f.puts mail.encoded.gsub(/r/, '')
        f.close_write
        sleep 1 # <---- added this line in order to give sendmail some time to process
      end
    end
  end
end

module Mail
  class Sendmail

    def initialize(values)
      self.settings = { :location       => '/usr/sbin/sendmail',
                        :arguments      => '-i -t' }.merge(values)
    end

    attr_accessor :settings

    def deliver!(mail)
      envelope_from = mail.return_path || mail.sender || mail.from_addrs.first
      return_path = "-f "#{envelope_from.to_s.shellescape}"" if envelope_from

      arguments = [settings[:arguments], return_path].compact.join(" ")

      Sendmail.call(settings[:location], arguments, mail.destinations.collect(&:shellescape).join(" "), mail)
    end

    def Sendmail.call(path, arguments, destinations, mail)
      IO.popen("#{path} #{arguments} #{destinations}", "r+") do |io|
        io.puts mail.encoded.to_lf
        io.close_write  # <------ changed this from flush to close_write
        sleep 1 # <-------- added this line
      end
    end
  end
end