@Ramesh,Thanks for posting in Q&A.
From your description, I know that you encountered an issue that you can forward the attachment from work account to personal account, even you can open the attachment, see the data and save it while the app applied app protection policy.
Based on my research, I find that when you save attachments on managed apps that applied app protection policy, they were encrypted, and you can read them. However, when you send an encrypted attachment via Outlook, Outlook will automatically decrypt the attachment and allow you to send it, read and save it.
Hope this can be helpful.
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