Add basic Postfix install through Manifest file
I will mention Sendgrid but it's same for other services like Mailgun, Amazon SES etc.
Setting up Sendgrid SMTP for use with ActionMailer is easy and works fine when the background workers are added to the mix. However it doesn't scale well in the long run. The best practice is to use e.g. Postfix mail transfer agent (MTA) and relay to Sendgrid. Rails can then simply use sendmail and all SMTP queueing and heavy-lifting is done by Postfix.
Here's the excerpt from the Sendgrid integration docs:
"A local mail server, such as Postfix, is the most robust way to send email through SendGrid when configured to queue all email from your application and then relay the messages through SendGrid as a smart host. This has the least latency from your application's perspective with the added benefit of handing your email off to a fault tolerant server. If internet connectivity between your servers and ours drops, a local mail server gracefully handles queuing and resending the email, as opposed to building that intelligence into your sending application.
Local mail servers also have advantages at high volume as they can use some of the more complex parts of the SMTP protocol, such as connection reuse and pipelining. With these techniques a mail server sends significantly more traffic in a given time than if you have individual scripts connecting for each message."