Summary: Fixes T5956. We changed the default mail encoding to `quoted-printable` to fix delivery via SendGrid via SMTP, but this broke multiple other mailers. - Change the default back to 8bit (which works everywhere except SendGrid). - Add a configuration setting for selecting `quoted-printable`. - Document this issue. - Discourage use of SendGrid in documentation. (IMPORTANT) @klimek @nickz This reverts the `quoted-printable` fix for SendGrid. You will need to adjust your configurations (set `phpmailer.smtp-encoding` to `quoted-printable`) and restart your daemons or mail will get double newlines again. Test Plan: - Sent mail via SendGrid with various `phpmailer.smtp-encoding` settings, saw mail arrive with specified encoding. Reviewers: btrahan Reviewed By: btrahan Subscribers: klimek, nickz, epriestley Maniphest Tasks: T5956 Differential Revision: https://secure.phabricator.com/D10397