Whitelist or Blacklist a Remote Sender in Zimbra Web Client
See also Route Incoming Messages to Folders
Whitelisting a remote sender will stop their mail from being (falsely) filtered by Zimbra. If you are having trouble receiving e-mail from a particular remote party, the first thing you should do is whitelist them.
You can whitelist or blacklist a single sender, or an entire domain.
Quick Add to White or Black Lists
Users can right-click on a message and quickly choose to block (blacklist) or allow (whitelist) the sender or entire sender domain.
Manage White and Black Lists
ZWC → Preferences → Mail → Spam Mail Options