In Zimbra mail initially mail lands in the proxy SMTP port and then is sent to the actual SMTP server and only after that the recipient verification is triggered. Below steps are useful to configure Sender address verification so that zimbra will immediately reject invalid user accounts with a 550 NDR
1. Login to the server and switch to zimbra user
2. Run the following commands one by one
zmprov mcf zimbraMtaSmtpdRejectUnlistedRecipient yes zmprov mcf zimbraMtaSmtpdRejectUnlistedSender yes
3. Restart the following services after running the above commands.
zmmtactl restart zmconfigdctl restart