Should anyone need a workround for this situation I have found something that works until I can implement something better.
Configure DKIM for main domain via Plesk interface
Enable DKIM for each domain alias via MailEnable managamnet console.
Make the selector the same as the domain name...
I have a main domain and a number of alias domains (34) for this main domain, each alias is used to show a branded version of the website. Each domain can send email using the alias domain address.
Each alias has:
Synchronize DNS zone with the primary domain: ON
Mail service: ON
Web service: ON...