Outlook asking for passwords since Fortigates installed.

[ICTSupport]

Hi,

I wasn't sure where to post this but figured here was the best place.

Every since we have had our Fortigate's installed we have started receiving random prompts on everybodys Outlooks in the office for the passwords for the mailboxes. It doesnt happen all the time and is very random when it ask's you. Sometimes it will be when you first start outlook, somestimes it will be in the middle of the day when reading an email.

All we do is click okay and it goes away and doesnt appear for several days again.

We are receiving all emails just fine and doesnt appear to affect anything except for our server email that we dont check often and runs 24/7 which will hang if we dont click okay.

Does anyone have any idea why this is happening  I have tried google to no avail and it has only been happening since the install.

We are very new to Fortigates/Fortinet so any help would be much appreciated.

EDIT: To calrify the Outlook is Office 2016 and not 365. We also dop not use exchange and simply use IMAP or POP mailboxes.

[tanr]

Are you doing deep inspection?  Is the backend server Office365 or one of the Exchange flavors?

If you're on 5.6.x or above you can set up specific security policies for the Internet Service for Office365 without deep inspection.