Attention
Check out the lightweight on-premises email archiving software developed by iRedMail team: Spider Email Archiver.
With iRedAdmin-Pro, please click menu in main navigation bar: `Add -> Mail Alias'. Screenshot:

cn=Manager or cn=vmailadmin.Create new entry here under
ou=Aliases of your domain, and select Default in right panel.mailAlias in right panel.WARNING: Attribute enabledService requires two values: mail, deliver.
enabledService: mail, deliver.accountStatus with value active. This is required.mailForwardingAddress.
You can add as many members as you want.Full LDIF data of a sample mail alias account:
dn: mail=myalias@mydomain.com,ou=Aliases,domainName=mydomain.com,o=domains,dc=iredmail,dc=org
objectClass: mailAlias
accountStatus: active
cn: Test Name
enabledService: mail
enabledService: deliver
mail: myalias@mydomain.com
mailForwardingAddress: user1@mydomain.com
mailForwardingAddress: someone@gmail.com
mailForwardingAddress: someone@hotmail.com