Consider the following scenario:
You're using Active Directory Federation Services (AD FS) for POP and IMAP client access authentication.
You have a claim rule to block access if the value of the X-MS-Client-Application claim type is not Microsoft.Exchange.PopImap.
These settings had previously worked as expected. However, you noticed recently that POP and IMAP clients fail to authenticate even though the correct user name and password is used.
Additionally, if you examine the AD FS access log, you may see entries that resemble the following:
[<Date><Time>] "POST /microsoftonline/ws-username HTTP/1.1" 403 ... "Microsoft.Exchange.Imap"
[<Date><Time>] "POST /microsoftonline/ws-username HTTP/1.1" 403 ... "Microsoft.Exchange.Pop"
A change was made recently in the service to separate POP and IMAP authentication. Instead of Microsoft.Exchange.PopImap, the value that's sent in the X-MS-Client-Application header is Microsoft.Exchange.Imap or Microsoft.Exchange.Pop.
In the existing claim rule, change the value of X-MS-Client-Application from Microsoft.Exchange.PopImap to Microsoft.Exchange.Imap and Microsoft.Exchange.Pop.
For more information, see the following resources: