Plesk 12.x / Linux :: Plus Addressing With Dovecot Does Not Work

I did the following configuration:

Postfix
in the /etc/postfix/main.cf I added the following configuration parameter
recipient_delimiter = +
Managed a sieve script in webmail which contains the following:
1: # Sieve Filter
2: # Erzeugt von Ingo (http://www.horde.org/apps/ingo/) (03.02.2015, 02:53:39)
3: require "fileinto";
4: # Plustest
5: if address :all :comparator "i;ascii-casemap" :contains "To" "+plustest" {
6: fileinto "INBOX.plustest";
7: stop;
8: }

I created a folder in my INBOX with the name plustest, now I sent me an email from an outside freemail-account to see if it works. In the logfile /var/log/maillog I can see the following entry:

Feb 3 15:06:25 mail postfix-local[18290]: cannot chdir to mailname dir <name>+plustest: No such file or directory
Feb 3 15:06:25 mail postfix-local[18290]: Unknown user: <name>+plustest@<dom.tld>
Feb 3 15:06:25 mail postfix/pipe[17708]: 0ECAF4C1B3: to=<name+plustest@dom.tld>, orig_to=<aliasname+plustest@dom.tld>, relay=plesk_virtual, delay=0.61, delays=0.52/0/0/0.09, dsn=2.0.0, status=sent (delivered via plesk_virtual service)

I changed my real name to <name> and my domainname to <dom.tld> in this post.