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dotCloud services are able to send E-mails out of the box. However, there is a high chance that the mail will never be delivered correctly due to anti-spam policies for mail coming from the “cloud”.
The best way to send mail is to use a third party service that will relay messages for you and ensure they are not viewed as spam. dotCloud uses (and recommends) MailGun for this purpose, but other options exist: the simplest is to use your GMail account, but the messages will look like they are sent from your GMail address. Other providers include SendGrid or CritSend.
The relay is configured from your dotcloud.yml, here is an example with MailGun:
www: type: php config: smtp_server: smtp.mailgun.org smtp_port: 587 smtp_username: email@example.com smtp_password: YourMailGunPassword
The config dictionary of a service is applied when your service is launched for the first time. If you change it, you will need to destroy the concerned service and push your application again to apply the changes.
You are also advised to use an internal dotCloud mail relay that will spool mail from your instances and forward them to the third party of your choice as explained in the SMTP service documentation.