ActionMailer works for gmail/yahoo, but not for other email addresses...HELP!

I have a rails app that can send emails to gmail, yahoo accounts. But
when it comes to more obscure accounts like email@my-app.com or
email@blah.edu, the mail never arrives even though the log shows that
the email was sent. Any ideas why this may be happening?

I have sent an email directly through the same web server through the
command line using

mail -s and that worked. Why doesn't it work through my rails app?

here are my configuration settings (mostly default)

# Disable delivery errors, bad email addresses will be ignored
  config.action_mailer.delivery_method = :smtp
  config.action_mailer.raise_delivery_errors = true
  config.action_mailer.perform_deliveries = true
  config.action_mailer.default_charset = "utf-8"

config.action_mailer.smtp_settings = {
    :address => "localhost",
    :port => 25,
    :domain => "my-app.com"
  }

help please!

thanks
Tony

help! anyone?