How to recover soft deleted mailbox

1) Connect to Exchange Online through Powershell.

If MFA is not enabled in your O365 environment, follow below URL:
https://docs.microsoft.com/en-us/powershell/exchange/exchange-online/connect-to-exchange-online-powershell/connect-to-exchange-online-powershell?view=exchange-ps

If MFA is enabled in your O365 environment, follow below URL:
https://docs.microsoft.com/en-us/powershell/exchange/exchange-online/connect-to-exchange-online-powershell/mfa-connect-to-exchange-online-powershell?view=exchange-ps

2) Run the following cmdlet:

Undo-SoftDeletedMailbox -SoftDeletedObject [e-mail]

Note: Deleted mailbox can only be restored through Powershell