We have not enough information about your mailflow so its difficult to
help. I had a similar issue and will tell you what i have done, maybe it
will help: In our company ServerX sends internal Mails to our Fortimail
and the Fortimail relays the mails...
I often simply use Firefox for this. On newer versions you can Press F12
and have a Debugger for websites.Under the ribbon Network you can see
all Network Communication and can also see when something is blocked:
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.