An upstream host is a host that forwards email to you, e.g., your ISP or external MX. If you get
inbound email from static upstream hosts, it is necessary that you enter the hosts here. Otherwise
spam protection will not work properly.
To add an upstream host either click the Plus icon or the Folder icon for drag-and-drop from the
Networks object list. How to add a definition is explained on the Definitions& Users > Network
Definitions > Network Definitions page. If you would like to only allow upstream hosts select the
checkboxAllow upstream/relay hosts only. SMTP access will then be limited to the defined
upstream hosts. Upstream hosts can authenticate to get relaying rights. ClickApply to save your
settings.