Are the messages landing in spam sent by a Fortinet product/service?
Is it the local/default smtp server you're using?
If you do, then you should move away from that, it's not intended to be used in production and messages will likely land in spam.
If you have your own server and messages still land in spam, then maybe your email server config is incomplete.
Aspects that could have been missed are DKIM, SPF.
Please read this, it will help:
DKIM, DMARC, SPF
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