A typical smtp.rb initializer looks like this:

ActionMailer::Base.smtp_settings = {
    :address              => 'smtp.domain.com',
    :port                 => 123,
    :domain               => 'domain.com',
    :user_name            => 'username',
    :password             => 'password',
    :authentication       => 'plain',
    :enable_starttls_auto => true
}

If you are sending out email from an SMTP server that does not require authentication, a common mistake is to set empty string in user_name and password. This is incorrect because the action mailer will try to authenticate with empty username and password.

ActionMailer::Base.smtp_settings = {
    :address              => 'smtp.domain.com',
    :port                 => 123,
    :domain               => 'domain.com',
    :user_name            => '',
    :password             => '',
    :authentication       => 'plain',
    :enable_starttls_auto => true
}

You should just remove the user_name, password and authentication key from the setting:

ActionMailer::Base.smtp_settings = {
    :address              => 'smtp.domain.com',
    :port                 => 123,
    :domain               => 'domain.com',
    :enable_starttls_auto => true
}