Hi @Sedat Özdemir ,
Thanks for sharing these information, I did a research about add-ins in Outlook mobile app, and I didn't found anything that could prove the add-ins could also working for Exchange on-prem mailboxes: Outlook mobile add-ins are supported on all Microsoft 365 business accounts, Outlook.com accounts, and support is coming soon to Gmail accounts.
And the first snapshot you provided also proved this.
Also I found a third-party article about Outlook add-ins: How To Use Outlook and OWA Add-ins
Outlook on iOS/Android. Note: Add-ins on Outlook for mobile devices can only work if the mailbox hosted under Office 365 solution.
Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.
Outlook client support, then I would think it is expected that the add-ins of Outlook mobile app could not working for on-prem Exchange :(
Regards,
Lou
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