Having set up a domain as mydomain.com (with the 'www' option ticked), mail sent to firstname.lastname@example.org was being rejected - although email@example.com was coming in just fine. The bounce error was
553 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1) (state 14).. Support staff suggested adding www.mydomain.com to /var/qmail/control/rcpthosts, although a Plesk upgrade would probably wipe out that file. I found out elsewhere that /var/qmail/control/morercpthosts is the proper place to add these extra acceptable recipient hostnames.
However, this all felt wrong; a manual fix for something that worked fine on other versions of Plesk?
I now realise what's going on. If I remove the 'www' checkbox prefix in 'Domain Administrator' for the domain, and add www.forandagainst.com as a domain alias, then the problem goes away. This appears therefore to be a failure on plesk 8.6's part to properly register the www alias for the domain, if it is checked in domain administrator.