That's a filesystem problem.
I figured as much.
The users do have distinct home directories (from passwd) separated from the mail location?
Yeah, in this case /home/mark is the home folder. If necessary I can remove the mail_location directive and test using the users home dir and see if that fails (also on ZFS but in the root storage pool).
is it possible that two Dovecot instances try to access the same storage ? Or is there some hardening (SELinux/ AppArmor) in action?
The dovecot mail system is running on OpenSolaris (OpenIndiana to be precise) and uses Solaris's Service Management (smf) to stop and start. I am relatively sure SMF won't let 2 instances run at once.
Or are there some special ACLs in ZFS that prevent that rename() operation on file system level? Does the user mark has the > permission at all?
Not certain. I don't think so and I certainly didn't enable anything along those lines but this is my first deployment on Solaris so I'm a little out of my depth. I'm used to Debian Linux.
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Mark