I've managed to surprise myself with some DNS configuration. This is the actual output of my terminal:
$ host www.sepa-converter.nl www.sepa-converter.nl is an alias for sepa-converter.herokuapp.com. sepa-converter.herokuapp.com has address 126.96.36.199 $ host sepa-converter.herokuapp.com sepa-converter.herokuapp.com has address 188.8.131.52
The result is that if you browse to www.sepa-converter.nl, you do not get the content served by sepa-converter.herokuapp.com.
I've used sepa-converter.nl as an alias for overopsepa.nl. And I want the mails to keep showing up at my Google Apps account. For that reason I've kept the A and MX records, but just changed the CNAMES to point to another server.
Why doesn't this resolve www.sepa-converter.nl to the right heroku app?
UPDATE: I understand that heroku doesn't return the same IP each time and I understand that that's not a problem. The problem is that the http requests serve content that is coming from a different dyno than the one aliased. If you check www.sepa-converter.nl against sepa-converter.herokuapp.com you'll see a different index.html being served.