One can configure the suffix for a custom Message-Id in Configure->Composer->Header->Use custom message-Id suffix. However that is a global setting for all identities and all SMTP resources. That is not optimal, in case you have multiple mail setups using different domains. You'd need to use a fantasy name for all domain then. But this then still could make it possible to find out that mails from different mail accounts are created by the same client. 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. Also see: Bug 457262 - Generate privacy-aware Message-Ids