A good addition to this list is https://postmarkapp.com/. They are meet EU law in regards to the GDPR. They don’t have a free tier for their relay service, but do have a nice free reporting tool for DMARC.
Can agree with this. Having slogged through the learning process of setting up DMARC and DKIM to supplement SPF, this site was a great help and allowed me to confirm my setup (Postfix and Dovecot) was behaving as expected and needed.
The only issue I have with a Postfix setup is providing an easy-to-use out of office auto-reply facility - I have tried the manual approach and also some scripted solutions but they don't seem to work reliably and, inevitably, I get comments from colleagues (it's a small, family-run business), that they just want to be able to turn on an auto-reply in Outlook (which is used as a front end for multiple inboxes on different mail services) and for it to 'just work'.
