Troubleshooting Tip: Outlook disconnecting when connected behind FortiGate

SAJUDIYA · ‎11-13-2024

Description

This article describes how to resolve an issue where the Outlook application gets disconnected after connecting behind FortiGate, but works if it is directly accessed.

outlook-disconnected (2).png

Trying to reinstall the Outlook application and following a solution from Microsoft does not resolve this issue:

There is no UTM profile or the SSL inspection profile is set as a no-inspection profile, but the issue still occurs.

Scope

All supported versions of FortiGate.

Solution

This issue is with the MSS value from client to server. Verify the MSS value by following the steps in this article.

Set the MSS value on the relevant Firewall Policy after verifying it from the article above (for example, MSS is 1350):

config firewall policy
    edit 1
        set name "test"
        set srcintf "port1"
        set dstintf "port2"
        set action accept
        set srcaddr "all"
        set dstaddr "all"
        set schedule "always"
        set service "ALL"
        set tcp-mss-sender 1350
        set tcp-mss-receiver 1350
    next
end

Now, it should connect.