Hi peraburek,
Have you checked if there are any issues with your DNS resolving?
Plesk 17.0.6 - DKIM - BUG report
if you enable DKIM in Plesk 17.0.6 and send test e-mail to test address found here http://dkimvalidator.com/
Code:Public Key DNS Lookup Building DNS Query for default._domainkey.MYDOMAIN.com Retrieved this publickey from DNS: v=DKIM1; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCy/lJG1ANBx+LTfnxMxykfzZQLkvqVCthS5P1GzHc/XKJlw2elZMd/2R8SNOexgpFXIcq2K6VdS0cvlq7crjZBmc/hCKDBh38mx48z3KZszGF4idjboJ/0dNQvJhKn//FU5KCyQ5LhVdVj47HMSDtC/I4gc6F9vcGga4KO7Fia3QIDAQAB; Validating Signature result = fail Details: bad RSA signature
Have you checked if there are any issues with your DNS resolving?