Hi all,
I’ve switched the mail provider that this Discourse instance uses to SendGrid. So far it looks like email delivery is fast, and more importantly all emails are delivered. Hopefully you will notice that email notifications and digests work reliably again too 
For most of this communities life we used Mandrill, which worked perfectly for us, but they changed their platform a few months ago and continuing with Mailchimp was cost prohibitive. So we switched to SparkPost which offered a lot for free but has been troublesome. For example, when we exceeded our email allowance a couple of weeks ago we were not sent any alerts. So, now we are with SendGrid and everything looks good again and the pricing plans are also a better fit for our needs 
Let me know if you see any improvement or issues.