Oh interesting, I thought that LMTP would be used instead for this case. I somewhat come from running WHM/cPanel servers whereby a website dispatching an email, without SMTP, would go through Exim using LMTP credential-less. Those emails got DKIM signed regardless of whether the account had any mailboxes or hosted emails at all. In terms of PHP this would be with sendmail. I noticed just now when sudo-ing into a terminal for a virtual server user that they don’t have permission to execute the sendmail binary, only root can do this.
Turning on the mail features for the virtual server doesn’t seem to cause any unwanted side effects, that’s fine by me. For the few clients that do login to Virtualmin we’ll just inform them that the mail features largely won’t work because they’re hosting them elsewhere. Though it appears you’re saying we’d need to make a mailbox user (eg “noreply@” or “website@”) within the virtual server for a website to ensure its dispatched emails get DKIM signed?
I’ve been gradually transitioning our servers to Virtualmin for a range of reasons (not just cost) and am not 100% clued up on the Virtualmin way of life quite yet. Most of my experience is with Exim either from WHM/cPanel or our pure Debian servers with only Webmin (but not Virtualmin).