The hyperlink in scheduled quarantine summary e-mails leads to an URL, which results in a "404 Not Found" message, reported by the FortiMail's web server after upgrading from FML 6.0.2 to FML 6.0.3
There is a bug for that in this firmware version 6.0.3 GA that will be fixed in next Firmware release, Bug ID 0522438.
For a workaround please give us ssh/https access to your Fortimail.
If that is not possible you can plan for downgrade to 6.0.2 GA.
Expected fix/update next week (46)!
FortiAnalyzer / 6.4.0
FortiClient / 6.2.6
FortiClient EMS VM / 6.2.6
FortiGate 300D HA 6.2.4
FortiGate 500E HA 6.2.4
FortiGate 30E / 60E / 100E / 6.0.9
FortiMail VM HA / 6.4.0
FortiSandbox VM / 3.2.0
FortiWeb VM / 6.3.2
FortiManager VM / 6.4.0