In order to set disk space limit for a domain from Mail enable server one need to perform below mentioned steps:
1) Login into MailEnable Professional MailEnable Admin area.
2) Go to MailEnable -> MailEnable Management -> Messaging Manager -> Post Offices.
3) Right click on domain for which one need to set disk space limits and click on all tasks -> set quotas.
4) Select the check box against set quota and enter the required number of disk space limit in a text box.
6) Click ok.