UPDATE: I finally resolved the issue. For how good Mautic is, I want to preface this with the fact that the Mautic + SendGrid integration documentation is extremely poor.
- You must install SendGrid via composer:
composer require symfony/sendgrid-mailer
- You must configure the Email DSN as follows:
Scheme: sendgrid+api
Host: api.sendgrid.com
Port: Leave Blank
Path: Leave Blank
User: Your SendGrip API key
Password: Leave Blank
I hope this helps prevent other people from wasting hours of their time.