You only need one ECOM100 IBox 710 per ECOM100 module, so you normally only need 1. You can have as many ECEMAIL IBoxes as you want (that fit within your Ladder memory).
Check out these FAQs on Host Engineering's website:
http://www.hosteng.com/FAQFiles/ECOM.htm#FAQ0105http://www.hosteng.com/FAQFiles/ECOM.htm#FAQ0059You can get access to all the FAQs for all of Host Engineering's products sold by AutomationDirect from
www.hosteng.com, then clicking on the FAQs link in the left navigation pane under Support.