Messages removed from users’ In Basket view are still retrievable for 60 days following removal (in this case, until September 13, 2026) before being purged from the system; see the FAQ blog post on how to retrieve. Afterward, all messages will still be retained in databases in case they need to be retrieved in the future for legal/privacy/etc. investigation, but messages cannot be retrieved this way for individual users.
Note that any messages that users postponed utilizing "Follow up" functionality are excluded from the auto-expiry rule until after the date the message comes back to the My Messages space.
For more information on In Basket, please see the CMIO's In Basket Management Guide: