Thanks to Ben Tasker
Assuming you’re running Postfix, it’s as simple as the steps below
First we make sure the virtual mappings file is enabled
vim /etc/postfix/main.cf
# Scroll down until you find virtual_alias_maps
# Make sure it reads something like
virtual_alias_maps = hash:/etc/postfix/virtual
# We also need to make sure the domain is enabled
virtual_alias_domains=example.com
Save and exit, next we add the aliases to our mapping file
nano /etc/postfix/virtual
# Forward mail for admin@example.com to jo.bloggs@hotmail.com
admin@example.com jo.bloggs@hotmail.com
Simple! And if we want to send to two different addresses at once, we just specify them
admin@example.com jo.bloggs@hotmail.com jos.wife@hotmail.com
Finally, we just need to create a hash (actually later versions of Postfix don’t require this)
postmap /etc/postfix/virtual
It’s exactly the same principle as passing mail into a local user’s mailbox.