I know that for individual users it is possible to do this using the Directory Utility.
But that's not what I want if I have to change dozens of users.
So I figured out a way to do this using dscl:
This is how you read the current value of the Mail Quota:
MailQuota=`dscl /LDAPv3/my.server.com -readpl /Users/$theUser MailAttribute kUserDiskQuota`
And this way you may set a new value:
dscl -u diradmin -P $diradminpass /LDAPv3/my.server.com -createpl /Users/$theUser MailAttribute kUserDiskQuota $newvalueInMB
Unfortunately this value does not get updated on the mail server immediately.
In order to get the mail server to update the quotas, you have to do this:
Delete or rename /var/db/.mailusersettings.plist
Then manually recreate the file '/var/db/.mailusersettings.plist':
serveradmin stop mail
rm /var/db/.mailusersettings.plist
touch /var/db/.mailusersettings.plist
chown _postfix:mail /var/db/.mailusersettings.plist
chmod g+w /var/db/.mailusersettings.plist
serveradmin start mail
Restarting the mail service updates the per user mail quota defined in OD.
What might also help is to try this:
doveadm quota recalc -u username
