There are a couple of ways, however I find from the listings page to be quick and easy as I can do several different property types quickly.
From your listings page choose "promote" tab (under listing)
Choose "View Prospect List" (second from bottom on drop down list)
Click red button to "Email Prospects"
For all types to be emailed, you need to click on each type (principals, brokers/others) and click email prospects. LoopNet has broken them down, but I have found more coverage of the properties is helpful.
Sep 10, 2015