In a hybrid deployment of Microsoft Exchange Online in Office 365 and your on-premises Microsoft Exchange Server environment, you create a new Office 365 mailbox in the Exchange Admin Center. To do this, you select Recipients, click Mailboxes, click New (), and then click Office 365 mailbox. However, when you take this action, email addresses aren't assigned to the mailbox according to the email address policies that are set.
By default, the Automatically update email addresses based on the email address policy applied to the recipient setting isn't enabled when you create an Office 365 mailbox in the Exchange Admin Center.
After you create a new Office 365 mailbox in the Exchange Admin Center, update the mailbox to use the email address policy. To do this, you can use either Exchange Management Shell or the Exchange Admin Center in the on-premises Exchange server.
Use Exchange Management Shell
Open Exchange Management Shell. For more information, go to the following Microsoft website: