To set up forwarding to an external address on any Exchange 2013 mailbox, you can go to the mailbox properties and configure mailbox features -> mail flow -> enable forwarding.
The problem is that the GUI will not allow you to enter a simple address to forward the mails to, but you can only configure a contact to which forward them (then you must have previously created it).
If you do not want to create a contact for each forward you need to configure you can configure everything via powershell with these commands, first you have to configure the forward "rule" on the individual mailbox:
Set-Mailbox -Identity username -DeliverToMailboxAndForward $true -ForwardingSMTPAddress email@example.com
Second step, we must enable autoforward globally that is disabled by default , check the status by executing this command:
Get-RemoteDomain Default | fl AllowedOOFType, AutoReplyEnabled, AutoForwardEnabled
To change the state and enable it:
Set-RemoteDomain Default -AutoForwardEnabled $true