Hi @Albrecht M ,
Looks like it has changed now.
For Exchange Online, the output of Get-MailboxDeviceMailboxPolicy shows that by default, all mail items can be synchronized to the mobile device:
And this document only mentions about the maximum of 1000 items per folder, while it doesn't say anything about the 30 days limit.
I've also checked my Exchange Online account in Outlook for Android and for iOS and can see ALL mails on both ends.
Sorry that I don't have an Exchange on-prem account to check it the same applies to Exchange om-prem as well, but I'd recommend running the command below in your on-prem environment to check the value of "MaxEmailAgeFilter":
Get-MobileDeviceMailboxPolicy | fl *EmailAge*
If it's already shows "All" but you still cannot see older mails on mobile devices, you can move to hybrid and based on the aforementioned information, there will be no issue when mailboxes are moved to Exchange Online.
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