Please advise if you are using the default email server notification.fortinet.net. Please be informed that the option to set reply-to has been disabled in newer version of FortiOS. It is implemented for resolving bug 751870:
Indeed, the feature is still available if you are using custom email server. The reply-to option is only not possible if you are using the default notification server from Fortinet. I've written an article explaining the respective, it should be made available once the article is approved:
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