Outside users attempting to send emails to our domain were blocked with the following error:
554 5.7.1 This message has been blocked because ASE reports it as spam.
I have extracted this info from Fortinet KB:
The AS Engine feature was released in FortiOS 4.0. A new AntiSpam Rule Set was also introduced in v4.0, this is the new rule which contains a Heuristic Antispam check for all suspicious spam upon passing through a firewall.
The AS Rule Set includes a predefined algorithm to scan the email including IP addresses and URLs contained in the message. If the email score is greater than the configured threshold then the email will be blocked. The default value is 80.
Note that if there are 2 items matched by the FDN DB, then it will increase its score to 200 (100×2). If the threshold is 120, it will be processed by spam action.
In order to disable the AS Engine feature, increase the threshold more than 100% as follows:
config system fortiguard set antispam-score-threshold 120 end
This configuration is a global and it cannot be set differently for each protection profile.