Configuring Postfix with other SMTP provider

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http://serverfault.com – I want to use SendGrid as my email sending service, but want to also use Postfix's internal queue mechanism to manage the emails sent through Sendgrid. So basically what I want to do is to configure Postfix to send emails through Sendgrid's SMTP, and I will configure my app to send the emails using the local Postfix. My question is, how can I configure Postfix to use an external SMTP? Looked here but didn't see anything useful. (HowTos)