problem e-mail delivery to addresses with domains configured in imscp, external no problem

Hello everyone,
I use imscp only for hosting while for e-mails I use external services.
The problem is when the php mail function sends to a domain registered in imscp and from the log of the mails it results "user unknow", therefore imscp searches the user locally.
Instead when I send to an external domain it works correctly.
How can I change this behavior and transmit all mail to the outside?
I tried to disable mail accounts with "-1" but it does not work.

thank you

I did not find anything in the forum but I found the solution.

first operation:
It is necessary as "administrator" to change the hosting plan and choose "external mail server" on "yes"

second operation
It is necessary to enter the "end user" account, that of the hosting user, choose in the upper right menu "mail", then in the menu on the left "external mail server" and finally "active"