There may be multiple reasons for that. One of the causes we see regularly is differences in the mail-from domain to the header-from domain.
Converia uses the email address of the registration/submission contact to send out emails from those modules. In most cases, the domain of these email addresses (e.g. contact@organizer.com) differs from the sending server (e.g. converia.server.de).
This divergency is due to technical reasons and very common in the field. Many recipients have aggressive spam filters active which is why domain differences in header-from and mail-from may be marked as spam.
To avoid this, we may set up a new email address using a Converia domain with an automatic forwarding to your email address. You may use this new email address for your registration/submission contact details and for mailings from Converia.
Please visit our manual for more information: Contact for Attendee Registration