@Marcel Talmaggies,Thanks for posting in Q&A.
From your description, I know you want to allow users to be able to download emails from the past 3 months by creating an Intune policy.
According to talking to Outlook engineers I learnt that you could enable Outlook's Use Cached Exchange Mode to keep emails in the exchange server to extend the retention time of emails, and also, we can also create a configuration policy and select Settings catalog to enable this setting through Intune, however, Intune can only enable this setting at the moment, and you can't specify how long you want to keep your emails.
If you want Intune to implement configure the length of time an email is saved, you can submit your feedback to Intune.
https://feedbackportal.microsoft.com/feedback/forum/ef1d6d38-fd1b-ec11-b6e7-0022481f8472
Thanks for your kind understanding.
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