@ Taurec42
The following commands have been executed for service account in addition to the security groups the service account resides in. Hope this helps.
[PS] C:>get-qaduser ServiceAccountName | get-qadmemberof | fl Name
Name : Domain Users
Name : Public Folder Management
Name : View-Only Organization Management
Get-MailboxDatabase -identity "DatabaseName" | Add-ADPermission -user aa1RME -ExtendedRights Receive-As, Send-As
Add-ADPermission -InheritedObjectType User -InheritanceType Descendents -ExtendedRights Send-As -User "ServiceAccountName" -Identity "DC=DomainName,DC=com"
Add-ADPermission -InheritedObjectType User -InheritanceType Descendents -ExtendedRights Receive-As -User "ServiceAccountName"" -Identity "DC=DomainName,DC=com"