How to fix a Wait condition error? - protractor

Question

I have a browser.wait() in an E2E test which is invoking a function defined in protractor.config.js file. When running the test, I get the following error:
Failed: Wait condition must be a promise-like object, function, or a Condition object
FYI - the function defined in protractor.config.js contains an If-condition which I need for two cases. When I remove the If-condition, the test runs fine. The function returns a: deferred.promise
What does this exactly means and how to fix it? Have been searching, but unfortunately can't anything related to it.
Function in protractor.config.js:
checkMail: function(user, subjectSent) {
const deferred = protractor.promise.defer();
var usermail;
var mailsubject;
var count = 0;
mailListener.on("mail", function(mail, seqno, attributes) {
var mailuid = attributes.uid;
var toMailbox = '[Gmail]/All Mail';
var i = ++count;
user = mail.to[0].address;
mailsubject = mail.subject;
if (i > 2) {
mailListener.stop();
return;
}
deferred.fulfill(mail);
});
if ((user === usermail) && (subjectSent === mailsubject)) {
return deferred.promise;
}
}
E2E it-function:
it("It should do xxxxxxx", (done) => {
browser.wait(browser.params.checkMail('user_email#yyyyy.com', 'Email subject'))
.then((email) => {
expect(email['subject']).toEqual("Email subject");
expect(email['headers'].to).toEqual( 'user_email#yyyyy.com' );
});
done();
});