The `SubscriptionForm` on the landing page broke when we moved the keyserver to the EC2 instance because the image didn't include `sendmail`. After installing `sendmail` it appeared that things were working and the requests were succeeding, but the emails didn't end up getting delivered reliably. There's additional configuration that's necessary to get the `sendmail` approach working reliably/consistently.
Instead, in this diff we move from the `sendmail` transport to using SMTP via nodemailer with Postmark as the "host."
These are some resources I referenced to possibly save reviewer some googling:
- https://postmarkapp.com/developer/user-guide/send-email-with-smtp
- https://www.mailgun.com/blog/email/which-smtp-port-understanding-ports-25-465-587/
- https://sendgrid.com/blog/whats-the-difference-between-ports-465-and-587/