The reason you're experiencing that is most probably (as far as I could see) due to a system called postgrey, which we use to combat spam.
Basically the moment you receive email form someone for the first time it is being automatically rejected. If the server is configured properly it will try to re-send the message in short period of time (eg. 5 min). Most spammers don't do that which prevents a very big chunk of possible spam hitting our mail service.
This means however that the first times you are expecting the email to arrive it will take few more minutes then normal. When receiving email form certain domains on regular basis, postgrey will put that domain on a whitelist, which means the emails will not be rejected/delayed.
However some email servers out there either aren't configured properly (even though sending legit emails) or resend emails from different email domains. Those domains need to be whitelisted manually.
If you still experience issues even though you received numerous email from one domain in the past, please send us (mail support_at_disroot.org) the list of such delayed emails addresses. We can then examine it more in detail and if needed, put that domain on a whitelist.