>>1- Open IIS
2- Right click on Default SMTP virtual server and select "properties"
3- click on "Access" tab
4- At the end of the opened page click on "Relay..." button
5- Select "All except the list below" radio button and check that there is no entery in the list below of the radio button
6- Click on the Check box below the ADD button to make it checked, the check box is following with this title "Allow all computers which successfuly autenticated ...">>

There are some instances that leaving the list of granted DNS/IP on Step 6 doesn't work. If it happens, put your servers IP address by clicking the add button..

Then, test it that works, configure your email clients SMTP: servername.dns suffix. e.g. dev.dalplus.com.bn (this must be the full computer name).

Regards..