Profile Manager not sending email invitations

After installing a new copy of OSX Server I started the Profile Manager and wanted to add some users and enroll them for the VPP Managed Distribution (I am already registered for VPP). In the VPP Managed Distribution section I hit "Resend invitation..." and a success message is displayed, but no invitation is received. The possible causes: - Missing SSL certificate - Misconfigured e-mail in OSX server - Misconfigured DNS I tried different email addresses and checked spam, but none of it helped. Why are my invitations not sent?