Mandrill configuration test mail won't send

My Mautic version is:
Linux vps0301 4.4.0-165-generic #193-Ubuntu SMP Tue Sep 17 17:42:52 UTC 2019 x86_64

My PHP version is:
PHP Version 7.0.33-11+ubuntu16.04.1+deb.sury.org+1

My problem is:
For some reason I can’t get Mandrill to work as a sending service. I’ve created the API key and edited the correct field in the configuration. When I push ‘Apply’ and go back to the Email Settings, the API key box is empty and when pressing ‘Send test email’ the error says ‘Sending failed for on or more recipients’. When I test a campaign it won’t work either. It says the action (send e-mail) is completed, but when logged in to Mandrill no sends are registered.

These errors are showing in the log:
[2019-10-18 11:50:46] mautic.ERROR: [MAIL ERROR] Sending failed for one or more recipients Log data: << @ => rejected

I’ve already tried clearing the cache, but that didn’t work.