Create only mail accounts when mail is actually enabled, instead of on domain creation
As soon as any account is created in Plesk, a mail account is setup in Smartermail, used for managing the mail domain. This is one of the few minor issues for those using Plesk in a load balanced solution (which is actually remarkably possible!). It is also tricky for those running a limited smartermail license (e.g. 250 accounts, many of which are now unwanted Plesk management accounts).
It should not be so difficult to setup the mail account only when mail is enabled on a domain, and have Plesk just leave the mail server alone for domains that do not have mail enabled.
I wrote some more here:
Simple Hosting GmbH commented
If only mail accounts are created when mailservice inside plesk is enabled for a domain would also solve the issue that if a domain uses external mail service locally mail accounts can not reach that domain because it's setup at smartermail but disabled.