If you want to have a single mailbox on Office 365 and be able to send as aliases of that mailbox, you will need to do some work around as it is not really support by Microsoft, see:

1 – Create Distribution List

  1. Create distribution group for the desired email address (ensuring is does not exist as an alias or otherwise in the tenant)
  2. Add desired destination mailbox as a member
  3. Open the Exchange Admin center
  4. Select “recipients” (side navbar) -> Select “groups” (top) -> Select the distribution group you just created, click the pencil icon to edit
  5. Select “group delegation” add your main mailbox user to the ‘Send As’ list
  6. Wait for approx 30 mins for Office 365 to provision the distribution list and update contact lists
  7. Optionally set up message rules in your mailbox to ensure emails to the distribution list email address are put into a specific folder

2 – Send As the distribution list via Outlook (Windows)

  1. In your Outlook client, create a new message
  2. If you cant see the From box, click ‘Options’, Click ‘From’
  3. Click on the now display ‘From’ dropbox and select ‘Other email address’
  4. Click on the ‘From…’ in the popup box
  5. Click on the ‘Offline Global Address List’, select ‘All Distribution Lists’, select your desired From address.

3 – Exchange Online

  1. Create new message
  2. Click the ellipsis to the right of the send button
  3. Right click on the from address, click remove
  4. Start typing the address you want to send from, select it from the drop down autocompleter