1. Install a new firewall rule set (copy smtp.xml to /etc/vmware/firewall)
2. Refresh Firewall ruleset (/sbin/esxcli network firewall refresh)
3. Activate the new rule set in the UI under network > firewall rules
I'm not sure if the words are correct; I have a german UI 🙂
Hint: This addional ruleset will be removed after a restart. You need to save the file on another place and copy it to the same place again (including a ruleset refresh).
Add steps/commands to the file /etc/rc.local.d/local.sh - that should solve the requirement too.
# Copy firewall rule SMTP
cp /vmfs/volumes/your_datastore/smtp.xml /etc/vmware/firewall
# Refresh firewall rules
/sbin/esxcli network firewall refresh
Example-Config of smtp.xml
<!-- Firewall configuration information for SMTP -->
Perhaps that answer your question?