Dovecot: Operation not permitted

I believe you ran into the same problem as I did:

Multiple UIDs
If you're using more than one UID for users, you're going to have problems running dovecot-lda, as most MTAs won't let you run dovecot-lda as root. Best solution is to use LMTP instead, but if you can't do that, there are two ways to work around this problem:

1. Make dovecot-lda setuid-root.
2. Use sudo to wrap the invocation of dovecot-lda.

Source: https://wiki2.dovecot.org/LDA#line-100

This is what I am doing after every update of the dovecot package:

sudo chgrp dovenull /usr/lib/dovecot/deliver
sudo chmod 04750 /usr/lib/dovecot/dovecot-lda
# optional: run postqueue to deliver queued mails immediately
postqueue -f

I'm running Ubuntu though, so you most probably will have to use a different group. I don't have a CentOS installation handy to look it up.

For completeness, similar problems also apply with multiple GIDs (possibly resulting in something like "Fatal: setresgid ...: Operation not permitted"):

If you need multiple uids/gids you'll need to set dovecot-lda setuid root or invoke it through sudo. See LDA#multipleuids for how to do this securely.

source: https://wiki2.dovecot.org/LDA/Postfix

Or use LMTP instead as mentioned in the other answer.