If you need to give Send As Permissions to a User in Office 365, it can be done using a few simple commands in Exchange Online PowerShell.
Send As permission is used when you need to give a user permission to use another recipient’s email address in the From address.
First connect to your Office 365 tenant, using Windows PowerShell.
To connect to Exchange Online, just start a PowerShell session and type in following commands:
$Cred = Get-Credential
The above command prompts you, for your Office 365 tenant admin credentials, it is important to enter as UPN format, such as e.g. firstname.lastname@example.org.
Next, we will create a new remote PowerShell session using the following cmdlet:
$s = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell -Credential $cred -Authentication Basic –AllowRedirection
Next we import of the PowerShell session:
You are now connected to Exchange Online with you Office 365 Tenant admin and can use Exchange Online PowerShell cmdlets available.
Next grant a user permission to access another user’s mailbox, so they have permission to open it:
Add-MailboxPermission <mailbox> -User <user or security group> -AccessRights FullAccess
Then give Send As permission to users, som they can send as the user:
Add-RecipientPermission <identity> -AccessRights SendAs -Trustee <user>
Reference documentation: http://technet.microsoft.com/en-us/exchangelabshelp/ff852815