Hey @Matthew1,
I’m sorry, but I have no experience with the FortiMail. The configuration snippet looks quite good to me. Just to be sure: the IPv4 address you garbled in the screenshot above is the one from the checkmk server, right?
Otherwise, the error output is a best practice:
Possible reason might be: Wrong credentials, wrong SNMP version, Firewall rules, etc.
Have you checked everything?