Setting Office 365 User with Send As permission

Connect to O365

$LiveCred = Get-Credential

$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic -AllowRedirection

Import-PSSession $Session

Send As

Add-RecipientPermission sendas@yourdomain.com -AccessRights SendAs -Trustee trustee@yourdomain.com

View all Send As permissions you’ve configured in your organization :  Get-RecipientPermission | where {($_.Trustee -ne ‘nt authorityself’) -and ($_.Trustee -ne ‘null sid’)}

Remove-RecipientPermission sendas@yourdomain.com -AccessRights SendAs -Trustee trustee@yourdomain.com