Mailroom deletes message from IMAP inbox, but does not add replies

Hi guys!

I am trying to set-up the reply-by-email feature in GitLab CE 8.13.1 on Debian 7 Wheezy. The mail environment (Postfix, Dovecot) is configured, set up and running. IMAP connection to localhost is used without sub-addressing. E-mails are delivered, gitlab-rake gitlab:incoming_email:check successfully verifies the credentials and connection ability.

When GitLab’s Mailroom is running, received e-mail replies get deleted from the inbox. (Not happening when it is not running, ergo Mailroom must be doing it.) However, the replies are not saved and they do not appear on the commit page.

This is my first time installing and using GitLab. Thank you in advance for any suggestions, what might be causing the problem.

Regards
Štěpán Glutexo Tomsa