You may have sen in a previous guide how we forward mail to external contacts in Exchange 2013. That it is quite easy to forward email internally and externally in Exchange 2013, what you may not have noticed is that by default, when you forward an email in Exchange 2013 when configuring it via the EAC then it does not by default leave a copy of the email in the original recipients mailbox.
To save a copy of the email in the original recipients mailbox in Exchange 2013 we need to run the command below.
Set-Mailbox -Identity "Allen" -DeliverToMailboxAndForward $true -ForwardingSMTPAddress email@example.com
By using the flag DeliverToMailboxAndForward $true wetell Exchange 2013 to not just forward but also deliver to the original mailbox.