https://docs.iredmail.org/sql.user.mail.forwarding.html (i put it for other forum members who might find your post later).
The principle behind forwarding is that by default the table "forwardings" contains a column named "address" which is the account mail address" and another column named "forwarding" which is the address to where the emails from this account are forwarded to.
By default the "forwarding" column contains the same address than the account mail address, which translate to "forward mail to original account mail address", which translate to "do not forward mail for this account"
So to forward the mail from one account to another, you just have to change the content of the column "forward" from the account you want to forward from and replace the original account mail address by the email address you want to forward to.
So let's take an example and say you wanna forward emails from account john to account anton:
select * from forwardings; (will display the mail accounts content from the 'forwardings' table so you see how it looks)
then we modify the line corresponding to john account to set the column 'forwarding' to anton's address instead:
update forwardings set email@example.com' where firstname.lastname@example.org';
select * from forwardings; (to show if the changes worked)
Then send 1 test email to john and see if anton received it
PS: this only works if users are in the same domain. If they are not, follow what's written in the documentation