Fix Exchange IMAP Migration Dates in OWA

Outlook on the Web (OWA) is the browser-based front end for Exchange Online and on-premises Exchange Server. After an Exchange IMAP migration, every email in OWA displays the migration date instead of the original received date. Why? Because OWA reads the same PR_MESSAGE_DELIVERY_TIME property that Outlook desktop uses, and that property was overwritten during migration.

When Exchange's built-in IMAP migration tool uploads messages to the destination mailbox, the transport pipeline treats each one as a new delivery. It stamps a Received header with the current processing time and sets the internal delivery timestamp accordingly. The original date information still exists in the Date header, but OWA never looks there for its date column display.

Some administrators assume the problem is limited to Outlook desktop and recommend switching to OWA as a workaround. (This advice comes up surprisingly often in migration forums.) It does not help. OWA and Outlook desktop both pull from the same Exchange message store. If the delivery time is wrong on the server, it is wrong everywhere - OWA, Outlook desktop, Outlook mobile, third-party EWS clients. There is no client-side escape from server-side date corruption.

Every email in OWA's message list shows the migration timestamp. The Focused Inbox tab, the Other tab, sent items, archived messages - all display the same wrong date. OWA's search functionality supports date filters like "received:this week" or "received:last month," but these filters query the corrupted delivery timestamp. A search for emails received in January 2024 returns zero results if the migration happened in March 2026.

OWA's conversation view still groups messages correctly by thread (using subject and message-ID references), but the date next to each message within a conversation is the migration date. Hovering over the date reveals the full timestamp - which is also wrong. The Filter feature for quick date-range filtering becomes unreliable. For organizations where OWA is the primary email client (common in environments that avoid desktop software deployment, or among remote workers on managed devices), this effectively destroys the chronological usability of every mailbox that went through the IMAP migration.