My problem does not lie with the sendmail-button, it is on, and the form does not work. When I uploaded the page it worked, when I tested it a month later it didn't. When I upload the page to a different server it also works, both sending directly to my gmail and to a domain address which is forwarded from here.
Someone told me it could have to do with my DKIM/DMARC/SPF/MX-settings. I have no idea what that means. Where do I check that?