Crashlytics - Testers not receiving email invite

From time to time we have problems with testers not getting an invite at all.

Since we are using Fabric to send mobile apps to our clients, this has become a huge issue. We found out that some email domains just don't receive the invitation, like our own company domain (and there is no filter on our end to block the invitation).

But currently there is a problem with the client not receiving the invitation, even if it is a hotmail address.

Why is this happening and how can we send these invitations properly? We really enjoy using the entire Fabric suit, so we don't want to just look for another beta.