Adding an email address in Server.app does not work

I'm running 10.11.5 with Server 5.1.5, and hosting multiple domains. When I add an email address to a user's account, that gets reflected by an entry in /Library/Server/Mail/Config/postfix/virtual_users . For most addresses, it works properly; however, in some cases, I'll add an address, and it appears in the "Email Addresses" list, but it never gets added to the virtual_users file. So, email delivery to that address fails. It's a real head-scratcher. Any ideas? I'd love to watch the back-end transactions taking place when I add an address to Server.app, but I'm not sure which logfile to watch.