One day i got a call in the morning, one of the users can’t access his mailbox from OWA (user is migrated from On premise Exchange to Office 365)

The error was like that

“something went wrong, The mailbox isn’t available. This may have occurred because the license for the mailbox has expired. To find out how to gain access to this mailbox again, contact the person who manages your email account.”

I have connected to the Exchange Admin Center on Office 365 , i found that this user has no licence but the mailbox exist !!!!!!!

After open a support case with MS Team , we found the below:

1- I have forget to add Exchange Licence to this user (oops……)  , while the mailbox is active and running for 30 days

2- Microsoft said that there is indeed a grace period of 30 days post mailbox move to Exchange Online within which license can be assigned.

3- The solution is to just add the license to the user and he will be able to access his mailbox again

This information is not available in any technet or any public article 

So for any one to avoid such issues , you can run the below command to get all users with mailbox and has no Exchange licence

get-mailbox | where {($_.recipienttypedetails -ne “Discoverymailbox”) -and ($_.skuassigned -ne “True”)} | ft name,skuassign*