Thinking about switching to Postmark because my transactional emails are now getting sent to client’s spam folders from Sendgrid. Sending from a custom domain
That happens often. That’s why most of us that rely on email to work use Postmark. 
SendGrid has been notorious for not working and having issues like that.
Is it possible to use both providers? I have a lot of workflows that use Sendgrid, can I test a few out with Postmark on the same app?
Yes, that’s what I did. Then once you get it working you can slowly transition.
Can anyone explain this ?