Weird issue with Mailbox size and Migration

Question

I have a mailbox that when you look at the size of the mailbox in outlook and in shell, it shows the mailbox to be 24gb in size. It was larger, but I had the user delete a ton of email in preparation for migration to O365 So now it shows to be roughly 24gb.

When I migrate the mailbox to a new database to compress all the whitespace, the migration size is over 40gb...even though outlook and exchange was saying the mailbox was only 24gb...AND after the migration, when you run something like Get-MailboxStatistics -Server Mail | Select DisplayName, ItemCount, TotalItemSize, TotalDeletedItems, Database | Sort-Object TotalItemSize -Descending

It shows the mailbox to be in the new database...but also in the old database...but only in shell. In ECP, it only shows the mailbox to be in the new database. So I migrated it again and it was showing to be in THREE databases. I thought this was an on prem issue, so I went ahead and migrated the mailbox to O365 and it's currently sitting at 32gb and still growing after 4.5 hours migrating even though its only 24gb in size

Even BitTitan was showing the mailbox to only be 24gb.

I'm at a bit of a loss here. I did check and verify the "TotalDeletedItems" was empty, so it's not the dumpster for the mailbox

Answer 1

Hi @Fink, Nathan ,

Deleted items are first retained in the database and then permanently deleted after the retention period.
You could clean up or delete items from the Recoverable Items folder to trim the mailbox size before doing a migration to EOL.

In addition, when a mailbox is moved to a different mailbox database, Exchange doesn't fully delete the mailbox from the source mailbox database when the move is complete. Instead, the mailbox in the source mailbox database is switched to a soft-deleted state.

Disconnected mailboxes | Microsoft Learn

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Answer 2

Try to resume the migration batch. If you cannot resume the batch, you can remove it and start over. Maybe due to cert expiration, you cant resume now.

Helpful links - https://learn.microsoft.com/en-us/exchange/mailbox-migration/manage-migration-batches