what do you mean by "only this email"? Do you have other mails and logs that just look fine?
It looks like the mail might have been stopped during transfer and hence not written.
Check the sender IP if this belongs to some spam range and what its IP reverse resolves to (nslookup thatIP). If you have the sender email address, you will also be able to take that domain name and look it up at some mail toolbox for blacklist lookups.
its from the company yokoy.ai, emails without attachmend or url we can recieve, but if they send attachmend (for example invite) the fortimail sends this email to quarantine and our user get a quarantine-message, but in the quarantine theres no email from this sender.
If i search in fortimail under monitor-log for this emails, i can see and if i open the Log from this emails, i see the errors
Actually I think it is not normal that this mail is quarantined and then not found in quarantine folder. Also the logs you shared I have never seen before on FML, they look like debug logs probably generated due to a bug, not related with regular filtering logs.
On the other hand please share session logs for of this quarantined mail, to check first why the mail was quarantined.
Then, depending on which check has quarantined the mail, you can either whitelist the source domain or create new profile/policy for the sender.
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