Support outgoing SMTP

Gmail and most other major mail providers no longer allow you to send from someone else's domain using their SMTP servers. This means most people only have the option of receiving email at their alum.recurse.com address, not sending from it.

Supporting this is a pretty low priority as it's a ton of work to get right, but I wanted to document this somewhere. It includes setting up SPF, DKIM, and DMARC, getting an IP address that's not on any spam blacklists (I think this is hard, but doable with EC2), monitoring deliverability, generating and storing random SMTP passwords, allowing users to rotate their passwords, etc.

Mailchimp and Basecamp have written some good guides on how to do this right:

- [Email Delivery for IT Professionals](http://static.mailchimp.com/web/guides/email-delivery-for-it-professionals/package/email-delivery-for-it-professionals.pdf)
- [Giving away the secrets of 99.3% email delivery
](https://signalvnoise.com/posts/3096-giving-away-the-secrets-of-993-email-delivery)

Alternatively, we might be able to get rid of postfix all together and use our email provider (Mailgun) for both sending and receiving emails. Mailgun has some amount of programmability when receiving mail, and I bet we could build the equivalent system either with their webhooks functionality or with their mailing lists). I think this would allow us to sidestep all the deliverability issues.