While most of the time you'll have no problems, increase in the varied email security systems out there and the need for security can sometimes stop or delay your email being delivered to certain email addresses.
Many systems (especially within corporations) can have overzealous security, which can cause delivery issues.
If you're sending in bulk to a particular company email address, we recommend getting firstname.lastname@example.org added to your whitelist.
We're currently working on a new feature, which will allow you to send from your own domain name. This works well visually and in some cases can improve delivery. It must be noted that in some cases this could hinder delivery too, all dependant on your domains reputation.
Also refer to our invite terms + conditions.