How to set up iptables to send mail from a rails application?

Question

Ruby on Rails

There is an application written in Ruby on Rails, through which emails are sent when a user registers.
Help, pliz, set up rules for iptables so that these messages are sent and reach the addressee.
Settings in config/environments/production.rb

config.action_mailer.smtp_settings = {
      address:               'smtp.gmail.com',
      port:                   587,
      user_name:             '[email protected]',
      password:              'mypass123',
      domain:                'mydomain.ru',
      authentication:        :plain,
      #:openssl_verify_mode  => 'none',
      enable_starttls_auto:  true
  }

Now all INPUT and OUTPUT policies are in DROP.
As soon as I do:

iptables -P OUTPUT ACCEPT
iptables -P INPUT ACCEPT

everything is sent (with one ACCEPT another DROP, could not set it up either).
Already like all the rules tried, and my ideas ran out. :(
Like those, iptables -A OUTPUT -p tcp --sport 587 -j ACCEPT
both on input and output, and on sport, dport and tcp, udp.
How to configure iptables, what rules to prescribe with DROP policies so that mail goes to recipients.