Help configuring Exchange Online mailbox on Outlook

Struggling to configure a specific user's Exchange Online (Plan1) mailbox on Outlook. I can login successfully to OWA,  https://outlook.office.com/owa/  Other users on the same domain have no problem, and their Outlook profiles were setup flawlessly. We have tried different versions of Outlook and can safely say the problem is not Outlook.  Any help would be highly appreciated.
iamdieterAsked:
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Jackie ManCommented:
We have tried different versions of Outlook

What versions have you tried?
Vasil Michev (MVP)Commented:
Any particular error? Can you pass the ExRCA connectivity/autodiscover test with that user? http://aka.ms/rca
iamdieterAuthor Commented:
Have tried both 2010 (SP2) and 2016.

Connectivity test is failing with SRV records which I have subsequently added. Still cannot configure Outlook
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Vasil Michev (MVP)Commented:
SRV records should not be a problem, considering all other mailboxes are able to connect. Moreover, the test should not even reach the SRV record check, autodiscover should be detected using one of the previous methods. Can you post the sanitized results here?
iamdieterAuthor Commented:
Vasil Michev (MVP)Commented:
Well it seems to be contacting your on-premises servers OK, getting the redirect value based on the user's targetaddress attribute OK (mellaartm@tourvest.mail.onmicrosoft.com), but then fails when contacting the O365 autodiscover endpoint.

You need to make sure that all the URLs and IP ranges listed here are allowed/accessible from your network: https://docs.microsoft.com/en-us/office365/enterprise/urls-and-ip-address-ranges?redirectSourcePath=%252farticle%252f8548a211-3fe7-47cb-abb1-355ea5aa88a2#exchange-online

Check your firewall/proxy config.

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iamdieterAuthor Commented:
We have tried it from 2 separate networks, still the same problem. The other Outlook clients that work, are behind the same firewall.
Vasil Michev (MVP)Commented:
Well, it might be just a misleading error message, but I cannot spot anything other in the log file. The only part missing is authentication, but if it works OK with OWA, that shouldnt be a problem. Do you perhaps use any form of 2FA on said account?
iamdieterAuthor Commented:
Not at all
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