On 10 Apr 2019, at 09:06, @lbutlr via dovecot <dovecot@dovecot.org> wrote:
> Should I add
>
> service stats {
>  unix_listener stats-writer {
>      user = dovecot
>  }
>  unix_listener stats-reader {
>      user = dovecot
>  }
> }
>
> to my dovecot.conf file?

I did this and it appears to have fixed the issue.

Also, the failed message strongly implies that the email was not delivered, since it happens on the delivery log line and there is not indication in the log that delivery succeeded. However, the message is delivered. It might be worth changing the message or still logging the actual message delivery?