One of the requirements was that he could read / write e-mail on his laptop, ipad and iphone. This looked like a very easy task but there was a hiccup. When I followed there guide to use IMAP on the Telenet mailbox it wouldn't work.
This happened because you need to enable IMAP. It was explained in article 2.
You can enable IMAP here.
Just enter the e-mail address as login and the corresponding password. You will receive an e-mail when IMAP has been enable (Not 2 days like they say in the first e-mail but between 5-60 minutes).
Then everything just worked fine.