A public folder in Office 365 Hosted Exchange will be assigned a @onmicrosoft.com address by default. And there is of cource no way of changing this to your primary domain.
Conncect to the Windows Azure Active Directory Module for Windows PowerShell using the following commands.
Create a placeholder for your credentials:
$LiveCred = Get-Credential
Popup box will ask for your Office 365 Global Administrator credentials.
Create a placeholder for your Powershell Session towards Exchange Online.
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic -AllowRedirection
Import your Powershell Session to connect to Exchange Online.
Run this command to allow all scripts:
Then run the following command to disable the Policy for assigning email Adresses in public folders.
Set-MailPublicFolder -Identity “\” -EmailAddressPolicyEnabled $False
Now, in your Office 365 tennant web admin go to Exchange – Public Folders – Highlight the public folder (email enabled) – Click the pencil (edit) – Click on Email Address – highlight the address you want to use as default – Click pencil – Check the default email address checkbox.
Save, close and test.