Replace Sendgrid SMTP to instead use their API

We're currently sending email via SendGrid's SMTP server endpoint instead of their API. According to SendGrid, their API is faster and more reliable than their SMTP relay. However, their API requires us to implement a webhook to receive events in case of failure to send email. Ensure that we have proper error handling to retry failed emails which adds a backoff configuration which adds a minor delay to ensure failed messages do not block other emails in the queue.