Because we encountered the same problem with Legacy-Outlook on the clients and apparently nobody delivers a working solution so far, although the problem is well known, I searched for documentation on the topic and found it at Microsoft:
I then took all the potentially used header fields - in the notation specified in the document - and inserted them into the default config as h1-case-adjust:
h1-case-adjust accept Accept
h1-case-adjust authorization Authorization
h1-case-adjust authrequired AuthRequired
h1-case-adjust cache-control Cache-Control
h1-case-adjust client-request-id Client-Request-Id
h1-case-adjust connection Connection
h1-case-adjust content-length Content-Length
h1-case-adjust content-type Content-Type
h1-case-adjust cookie Cookie
h1-case-adjust date Date
h1-case-adjust host Host
h1-case-adjust persistent-auth Persistent-Auth
h1-case-adjust pragma Pragma
h1-case-adjust request-header Request-Header
h1-case-adjust response-header Response-Header
h1-case-adjust server Server
h1-case-adjust set-cookie Set-Cookie
h1-case-adjust status-code Status-Code
h1-case-adjust transfer-encoding Transfer-Encoding
h1-case-adjust user-agent User-Agent
h1-case-adjust www-authenticate WWW-Authenticate
h1-case-adjust x-anchormailbox X-AnchorMailbox
h1-case-adjust x-clientapplication X-ClientApplication
h1-case-adjust x-clientInfo X-ClientInfo
h1-case-adjust x-content-type-options X-Content-Type-Options
h1-case-adjust x-deviceinfo X-DeviceInfo
h1-case-adjust x-elapsedtime X-ElapsedTime
h1-case-adjust x-expirationinfo X-ExpirationInfo
h1-case-adjust x-feserver X-FEServer
h1-case-adjust x-mapihttpcapability X-MapiHttpCapability
h1-case-adjust x-pendingperiod X-PendingPeriod
h1-case-adjust x-powered-by X-Powered-By
h1-case-adjust x-requestid X-RequestId
h1-case-adjust x-requesttype X-RequestType
h1-case-adjust x-responsecode X-ResponseCode
h1-case-adjust x-serverapplication X-ServerApplication
h1-case-adjust x-starttime X-StartTime
h1-case-adjust x-user-identity X-User-Identity
in addition to
option h1-case-adjust-bogus-client
in the frontend config.
And - by magic - it works.