KMail by default uses the local host name as suffix for Message-Id. This has two downsides:

1. It reveals the local hostname even in case a privacy aware mail server removed the first Received: header.
2. It is not guaranteed to be unique.

I'd not mind the second disadvantage as there is no easy way to make the second part unique to begin with, unless you generate some kind of UUID for that. But doing so might have other privacy implications.

A good default might be to use the domain name from an SMTP transport. Or the domain part of the mail address in an identity.

This bug could also be in a different product/component like Akonadi / Mail dispatcher agent, but it really depends on where this configuration option would best be implemented.