what I usually do if a have only few then model is where I usually put them, in case I have several then I create a directory called mailers each email gets a dedicated class for example you might have WelcomeMailer, or VerificationMailer.
<?php
namespace app\mailers;
class WelcomeMailer {
public function send()
{
}
}
// some controller or model
$welcomeMailer = new app\mailers\WelcomeMailer;
$welcomeMailer->send();
I put each mailer in a separate class because IMO it is more flexible approach, if my mailers get complicated i can break it down to several methods and test them, at the same time keep the logic in one place.