Problem
There is no mail quota configured but user/s receive a mail quota warning.
Analysis
In order to have no mail quota a user must have the attribute univentionMailUserQuota set to 0. Check all your users for that attribute.
To display all mail users without univentionMailUserQuota attribute might use the following LDAP search
root@ucs:~# univention-ldapsearch -LLL "(&(objectClass=univentionMail)(!(univentionMailUserQuota=*))(!(sn=none))(!(univentionObjectFlag=hidden)))" \
dn | sed '/^$/d;s/dn: //'
Solution
Add the univentionMailUserQuota attribute to all needed users
udm users/user modify --dn "<USER-DN>" --set "mailUserQuota=0"