Sigh... evidently, of all the thousands of system admins at LQ, I am the first one to create virtual mailboxes with postfix.
Fortunately I was able to figure it out on my own, or partly so.
1. I found a postfix tutorial for Gentoo:
http://www.gentoo.org/doc/en/virt-mail-howto.xml. It indicated that a vmail user should be created with
Code:
useradd -d /home/vmail -s /bin/false -m vmail
2. This isn't actually necessary because postfix will create the directories for you. Or, at least, it did for me. I had set virtual_mailbox_base = /var/mail/vhosts, and the /var/mail directory was already created by the system. The virtual mailbox directories won't be created immediately, but as soon as mail arrives for a domain, it creates the appropriate directories and assigns them to vmail:vmail with appropriate permissions.
3. This worked out a little differently then I expected, because the myorigin variable was set to $mydomain, so sendmail appends the domain name to mail normally directed towards root. So, I just added another entry to my vmailbox map pointing from root@<mydomainname> to my personal account. And this seems to work fine, at least according to my sendmail tests.