You can use same settings with Postfix. For example, if you use "alias_maps = hash:/etc/postfix/aliases" in old mail server, you can use same setting in iRedMail.
Okey, I added a new string to /etc/postfix/aliases in my new server with iRedMail:
After it, I run a command "newaliases".
There are two lines in main.cf:
alias_maps = hash:/etc/postfix/aliases
alias_database = hash:/etc/postfix/aliases
So, everything looks fine, but how to trigger this alias (smspager)? I tried to make a forward for a mailbox to "smspager", but iRedAdmin doesn't allow it. I even tried to change mailForwardingAddress parameter directly in ldap, despite this I always see in postfix log: status=bounced (user unknown)