Hi Jens,
Thanks for your suggestion. I'm pleased to advise we were already working on the ability to disable email sending for v8.1 (a request from our support team as it happens).

And of course you can always add yourself as a client account and send the email to yourself before perming a mass mail.

Version 8.1 is currently undergoing public pre-release testing. Please use this opportunity to test it out and share your feedback with us: https://whmcs.community/forum/533-whmcs-81-beta-discussion-board/
Feature Documentation: https://docs.whmcs.com/Disabling_Outgoing_Mail
To learn more about the beta and to take part, visit https://beta.whms.com