I'm trying to get blow Sieve filter to work

require ["fileinto", "imap4flags", "mailbox", "body", "envelope", "vnd.dovecot.pipe", "variables", "vnd.dovecot.execute"];

if envelope :matches "To" "*@*" {

  set "recipient" "${0}";

  set "user" "${1}";

  set "recip_domain" "${2}";

}

if envelope :matches "From" "*" {

  set "sender" "${0}";

}

#Check if recipient is valid user

if execute :output "valid_user" "user-verification" "${recipient}" {

    if string :matches "${valid_user}" "True" {

      if body :raw :contains ["message/notification"] {

        setflag "\\Seen";

        fileinto :create "Notifications";

        stop;

      }

    }

}

Where user-verification is an extprogram which calls API and verify user based on email address then returns boolean (as an output to the console).

Everything works fine when I will remove if string :matches "${valid_user}" "True"statement other way it looks like is not recognizing valid_uservariable.

When I pipe valid_user to some script just to capture value for that variable it throws an error:

error: specified :args item `True?' is invalid.

Why question mark was added to the variable in this case?

Thoughts?