FortiGate 80F - OnPrem Exchange Server

Hello everyone,

We have an OnPrem Exchange Server, it works via Fortigate as well. We only installed Fortigate about 6 weeks ago. Now we have the problem that access via OWA is not working properly.

The OWA access and the page itself are fine, you can log in, see the email and click on it. However, the preview of the email is not displayed. But you can send a new email. Internally, i.e. via localhost\owa, it works without any problems.

We are a bit confused, but suspect that Fortigate is the problem, as it worked before and also works via local. We have basically set up a virtual server in Fortigate and then a firewall policy.

Does anyone have experience with Fortigate and a local Exchange Server? Emails via Outlook work perfectly, but these problems only occur with OWA.

Do you need more informations?

Thank you in advance for your time.