To make an Exchange Distribution Group externally accessible:

Exchange Management Console → Recipient Configuration → Distribution Groups → Properties Mail Flow Settings → Delivery Restrictions → De-Select All Senders are Authenticated

Enable-DistributionGroup -Identity "Group Name"

http://exchangeserverpro.com/exchange-2010-grant-send-behalf-permissions-distribution-group/

When testing, pick the From address from the GAL.

This seems to require these steps:

1. Create a separate Distribution Group for each outbound identity
2. Set the Reply Address to the desired outbound identity
3. Modify the Distribution Group to grant SendOnBehalfOf permission to users
4. Modify the Distribution Group to grant SendAs permission to the same users
5. Restart the Information Store service (only needed to immediately activate the changes)

http://exchangeserverpro.com/exchange-2010-send-as-permissions-distribution-group/

• SendAs another user or public folder may be easier
• Consider using a security group rather than giving permissions to individual users (over and over)
• The Distribution Group must be a Universal group
  • Set-Group “Group Name” -Universal
• Restart the Information Store service to activate changes immediately (otherwise you have to wait)
  • net stop MSExchangeIS
  • net start MSExchangeIS

List Public Folder or Distribution Group:

Get-DistributionGroup "Group Name"

Optionally, you can further verify “Send As” target is a Distribution Group:

get-recipient -results unlimited | where {$_.emailaddresses -match "targetemailaddressg@yourdomain.tld"} | select name,emailaddresses,recipienttype

Show “Send As” permissions for a Distribution Group:

Get-ADPermission -Identity "Group Name" | where {($_.ExtendedRights -like "*Send-As*")} | FT -Wrap

Grant a user or group “send as” permission:

Add-ADPermission -Identity "Distribution Group Name" -User "User or Security Group" -ExtendedRights Send-As

Show “send on behalf of” permission:

Get-DistributionGroup -Identity "Distribution Group Name" | fl GrantSendOnBehalfTo

Grant a user or group “send on behalf of” permission:

Set-DistributionGroup -Identity "Distribution Group Name" -GrantSendOnBehalfTo "User or Group Name"