Allow mail_domains without local delivery (sending only)

Usecase: A webserver hosting the website example.com, but not the mail for that domain. And that website would like to sent out DKIM signed mails.

Should we enable the mailserver service in ispconfig on this webserver? I guess so. I've solved it in the past with a standalone opendkim server

When I create a mail_domain on that server it has the drawback that Postfix thinks it's responsible for the domain. So if the website were to sent a mail to a user of the same domainname it would not go out to the MX.

related: https://forum.howtoforge.com/threads/separate-smtp-send-only-server-in-ispconfig-multiserver-setup.89994/#post-459949

There is an additional usecase. When a domain migrates it's mail hosting away from our ISPconfig server but the client would like to keep the mailboxes around for a few months as a backup/archive ... When I mail then from the same server it does not go out to their new MX... If I disable the mail_domain to prevent that it blocks access to their mailboxes. Being able to uncheck the 'Local delivery' checkbox solves this.