For a variety of different reasons, sometimes there is the necessity of users or administrators to automatically forward e-mails addressed to one mailbox to another mailbox. To achieve this there are several options such as Outlook Inbox Rules or Transport Rules. Another option is to configure the mailbox itself using the ForwardingAddress setting which can be configured using the Exchange Management Console/Exchange Admin Center.
However, in some cases the end recipient is a SMTP address external to the local Exchange environment. As ForwardingAddress only works with local recipients, we can create a contact for the target mailbox and then use ForwardingAddress with the contact.
But creating a contact might not be desirable, so a more “direct” way is to use the ForwardingSmtpAddress setting instead (which can only be configured through the Shell). As such, what most administrators do is something like this:
Set-Mailbox nuno -ForwardingSMTPAddress mota@somedomain.com -DeliverToMailboxAndForward $True
After doing this, the administrator notices that e-mails are still not being automatically forwarded... Let us see why.
Within Exchange and Office 365, administrators can create remote domains. By default, every Office 365 tenant leverages the “*” domain. When a specific remote domain does not exist, the “*” remote domain setting are applied to the message. However, this is not true for on-premises Exchange environments where, by default, there are no remote domains.
A property of a remote domain is the AutoForwardEnabled property, which allows administrators to define if auto-forwarding is allowed on e-mails destined to the domain specified. By default auto-forwarding in Office 365 is allowed to all domains, but not in on-premises environments.
This means that administrators need to create a remote domain for the forwarding addresse's domain (or a generic one), in my example, somedomain.com, and ensure AutoForwardEnabled is set to $True (which it is by default when creating a new remote domain):
New-RemoteDomain -Name ExternalDomain -DomainName somedomain.com
Get-RemoteDomain ExternalDomain | Select DomainName, AutoDorwardEnabled
Note that using ForwardingAddress is a way for an administrator to bypass forwarding settings on remote domains, meaning it is not necessary to create a remote domain for the forwarding to work.
Bottom line: if you want to use the ForwardingSmtpAddress setting, make sure you have a remote domain for that SMTP addresse's domain with AutoForwardEnabled set to $True.
