In order to set SMTP connection limit for a domain one needs 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 the SMTP Limits and click on properties.
4) Go to restrictions tab.
5) Select the check box against Limit Maximum SMTP recipients and enter the required number of connection limit in a text box.
6) Click on Apply and ok.