Update on this: testing with https://www.mail-tester.com did show some issues with our spf records.
We use Google Apps and UserVoice and already had an spf record that was formated like this:
v=spf1 a mx include:_spf.google.com include:smtp1.uservoice.com ~all
Adding support for discourse generated the following error when checking with mail-tester:
Maximum DNS-interactive terms limit (10) exceeded
Removing a mx got us a score of 10/10 for our email test. It seems that it was something that was suggested by UserVoice documentation but not mentioned by Discourse or Google’s doc.
Maybe a DNS expert can give more information to make this useful for ppl running into a similar issue.