can't send email over http(s) proxy

Currently, GitLab does not support any type of proxy for SMTP. If a customer is in an environment where they must use a proxy to connect to external services (cloud provider, for example, needing to connect to an external service like Google GSuite to send mail).

We may be able to use an add-on such as https://github.com/camertron/net-smtp-proxy to make this work. More investigation would be needed, though.

Net::OpenTimeout: execution expired
	from /opt/gitlab/embedded/lib/ruby/2.3.0/net/smtp.rb:542:in `initialize'
	from /opt/gitlab/embedded/lib/ruby/2.3.0/net/smtp.rb:542:in `open'
	from /opt/gitlab/embedded/lib/ruby/2.3.0/net/smtp.rb:542:in `tcp_socket'
	from /opt/gitlab/embedded/lib/ruby/2.3.0/net/smtp.rb:552:in `block in do_start'
	from /opt/gitlab/embedded/lib/ruby/2.3.0/timeout.rb:101:in `timeout'
	from /opt/gitlab/embedded/lib/ruby/2.3.0/net/smtp.rb:551:in `do_start'
	from /opt/gitlab/embedded/lib/ruby/2.3.0/net/smtp.rb:521:in `start'
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/mail-2.7.0/lib/mail/network/delivery_methods/smtp.rb:109:in `start_smtp_session'
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/mail-2.7.0/lib/mail/network/delivery_methods/smtp.rb:100:in `deliver!'
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/mail-2.7.0/lib/mail/message.rb:2160:in `do_delivery'
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/mail-2.7.0/lib/mail/message.rb:260:in `block in deliver'
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/actionmailer-4.2.10/lib/action_mailer/base.rb:543:in `block in deliver_mail'
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/activesupport-4.2.10/lib/active_support/notifications.rb:164:in `block in instrument'
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/activesupport-4.2.10/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/activesupport-4.2.10/lib/active_support/notifications.rb:164:in `instrument'
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/actionmailer-4.2.10/lib/action_mailer/base.rb:541:in `deliver_mail'
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/mail-2.7.0/lib/mail/message.rb:260:in `deliver'
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/actionmailer-4.2.10/lib/action_mailer/message_delivery.rb:85:in `deliver_now'
	from (irb):2
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/railties-4.2.10/lib/rails/commands/console.rb:110:in `start'
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/railties-4.2.10/lib/rails/commands/console.rb:9:in `start'
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/railties-4.2.10/lib/rails/commands/commands_tasks.rb:68:in `console'
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/railties-4.2.10/lib/rails/commands/commands_tasks.rb:39:in `run_command!'
	from /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/railties-4.2.10/lib/rails/commands.rb:17:in `<top (required)>'
	from bin/rails:9:in `require'
	from bin/rails:9:in `<main>'