I have been exploring dovecot and came across this:
$ sudo doveadm mailbox list
doveadm(root): Error: Mail access for users with UID 303 not permitted (see
first_valid_uid in config file, uid from mail_uid setting).
Hmm. After setting "first_valid_uid = 300":
$ sudo doveadm mailbox list
doveadm(root)<4380><4VP8EENer2McEQAAvbJltg>: Error: chdir(/root/) failed:
Permission denied (euid=303(vmail) egid=303(vmail) missing +x perm: /root, dir
owned by 0:0 mode=0700)
INBOX
I do not understand the error message.
There is an important hint in the error. (root). You might want to add -u someuser there?