1. go into the subscription and select the mail register. in the head line you will find the button "change settings". there you will find what do if receiving mails for non existing accounts. here you can specify a mail adress. so you can forwarding everything from *@a.com to [email protected].
2. but maybe you just need to create domain b as "domain alias" of domain b!? so all mail accounts of domain a exists for domain b also.
Thank you Martin, but i want to forward any account from domain a.com in Plesk to b.com elsewhere. Creating a domain alias only works if both domains are in the same server.