Why doesn't dovecot create a quota file?

Question

Dovecot

Loading the plugin:

plugin {
  quota = dict:user::file:/var/vmail/%n/.quotausage
  sieve=/var/vmail/%n/.sieve
}

I connect and look at the logs:

2015-09-23 15:34:26 pop3: Debug: Loading modules from directory: /usr/lib/dovecot/modules
2015-09-23 15:34:26 pop3: Debug: Module loaded: /usr/lib/dovecot/modules/lib10_quota_plugin.so
2015-09-23 15:34:26 pop3: Debug: Added userdb setting: mail=maildir:/var/vmail/usertrelay/
2015-09-23 15:34:26 pop3: Debug: Added userdb setting: plugin/quota_rule=*:storage=20B
2015-09-23 15:34:26 pop3: Debug: Added userdb setting: plugin/sieve=/var/vmail/usertrelay//.sieve
2015-09-23 15:34:26 pop3(usertrelay): Debug: Effective uid=5000, gid=5000, home=/var/vmail/usertrelay/
2015-09-23 15:34:26 pop3(usertrelay): Debug: Quota root: name=user backend=dict args=:file:/var/vmail/usertrelay/.quotausage
2015-09-23 15:34:26 pop3(usertrelay): Debug: Quota rule: root=user mailbox=* bytes=20 messages=0
2015-09-23 15:34:26 pop3(usertrelay): Debug: Quota grace: root=user bytes=2 (10%)
2015-09-23 15:34:26 pop3(usertrelay): Debug: dict quota: user=usertrelay, uri=file:/var/vmail/usertrelay/.quotausage, noenforcing=0
2015-09-23 15:34:26 pop3(usertrelay): Debug: maildir++: root=/var/vmail/usertrelay, index=, indexpvt=, control=, inbox=/var/vmail/usertrelay, alt=
2015-09-23 15:34:26 pop3(usertrelay): Info: Disconnected: Logged out top=0/0, retr=0/0, del=0/9, size=7044964

but the file /var/vmail//usertrelay/.quotausageis missing.
Why is it not created?
Everything was done according to mana
UPD: I
connected via IMAP and the file was created. And how to make for POP3?
config says:

protocol pop3 {
  mail_plugins = quota
}