Sieve and output variable for execute

[Adam Raszkiewicz]

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?

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