I subscribed office 365 business plan (compare office 365 enterprise) single user. Then I thought how to create a professional support mail id without subscribing another user licence. Then shared mailbox concept came into picture. This post describes how to create shared mailboxes office 365.
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Add domain to office 365 and office 365 dns records
This post describes about how to Add domain to office 365 and office 365 DNS records. You can follow the below steps in office 365 admin center.