I read up on using relayhost with Postfix 2.6.6. It's possible to use different relay hosts, with different credentials, for different senders. But how do I use direct delivery with specific senders (in this case, only one sender), while all other mail gets sent via a single relay host/credential (in this case, Sendgrid).
Background: all mail is transactional and generated by web applications on different domains on the LAMP server where Postfix runs. No MX records for any of these domains points to this server (they are mostly google apps). All domains, except one, sucessfully use the sendmail>postfix>sendgrid>remote SMTP route. This one domain/application needs to send directly (sendmail>postfix>remote SMTP) in order to implement better delivery tracking. I don't want to use direct delivery via SwiftMailer itself, because it introduces latency in the application's mail processing. I want SwiftMailer to use the
Swift_SendmailTransport and deliver via Postfix, and then we will comb the Postfix logs for delivery failures.