For using the paid Proton Mail, I Encountered the problem while setting up the DKIM section. Error message was "Cannot set CNAME Record and MX Record together." Solution from dynadot tech support was to use the "Subdomain Records" . Example "Subdomain" = "protonmail._domainkey ; "Record Type" = "CNAME" ; "IP Address or Target Host" = "protonmail.domainkey.xxxxxxxxxxx...."