Попробуйте установить для enable_starttls_auto значение false в настройках smtp_settings:
ActionMailer::Base.delivery_method = :smtp
ActionMailer::Base.raise_delivery_errors = true
ActionMailer::Base.default_charset = "utf-8"
ActionMailer::Base.smtp_settings = {
:address => 'server.com',
:port => '25',
:domain => 'yourdomain.com',
:user_name => 'username',
:password => 'password',
:authentication => :login,
:enable_starttls_auto => false
}