How to Merge 2 O365 mailboxes

Hello RoseBJ,

Thanks for your post in Microsoft community. To better help you, we also use the same email addresses (i.e. first.last@domain1.com and first.last@domain2.com) in our suggestion. Please check it.

A. To keep all the users old mail :

Option 1: Give Full access delegate permission to other domain2 user and access domain1 user account from Outlook. For this you need to open first.last@domain1.com user account from Microsoft365 Exchange Admin center and give full access mailbox permission to user account first.last@domain2.com. Due to Auto-mapping feature, first.last@domain1.com mailbox reflect in user first.last@domain2.com Outlook. Note: Delegated account also access from Outlook Web app.

• To do so: login to Microsoft365 Admin center >  Exchange > Recipients > select your user account (i.e first.last@domain1.com) to edit his information > Click on Mailbox Delegation > click (+) icon under Full access and select other user account (i.e first.last@domain2.com) to add > click ok and then click on Save to change the effects.
• Now due to auto-mapping features, when delegated user account  (i.efirst.last@domain2.com), open his Outlook, the other mailbox automatically added in his profile and he can check and access all mails.
• You may refer this article for Accessing other people’s mailboxes in Office 365

Option 2: Use content search tool to back up entire mailbox data of user first.last@domain1.com. After a search is successfully run, you can export the search results to a local computer. When you export email results, they can be downloaded to your computer as PST files or as individual messages (.msg files). Then with import or open data file option in Outlook, add such back up .pst file in your user’s account first.last@domain2.com. Backup data reflects in same folder vise as it was in first.last@domain1.com.

• For step by step detail information, please check Content Search.

B. Now for going all emails in to domain2.com, create/enable forwarding rules user vise.

• To do so: login to Microsoft365 Admin center >  Exchange > Recipients > select your user account (i.e first.last@domain1.com) to edit his information > go to Mailbox Features > Click on Mail flow, view details and Enable forwarding , then click on Browse tab and select your account to whom you wants to forward your email. (note: if you want to Deliver messages to both forwarding and address mailbox, please enable this option).
• Article for your reference : Configure email forwarding

Thanks and regards,

Ankita Vaidya