So I have used Sendgrid, Mandrill, SparkPost and SES in production and dabbled with Mailgun for side projects. The biggest issue I have is with Logs where other than Mandrill, no one shows the actual Body of the email in logs. For our purposes, we need that. Do you log the Email body ? If yes, I will give it a shot.
Are you stating you want the email body indefinitely though? Does Mandrill store this? I'm yet to fully check them out.