Different SMTP based on domain

I believe what you need is the phpmailer_init action hook.

Here is an something you can do.

add_action( 'phpmailer_init', 'custom_smtp_settings' );

function custom_smtp_settings( $phpmailer ) {

    $phpmailer->isSMTP();

    switch ($_SERVER['HTTP_HOST']) {

        case 'example1.com': //name only, no http
            $phpmailer->Host="smtp.example1.com";
            $phpmailer->SMTPAuth = true; // Force it to use Username and Password to authenticate
            $phpmailer->Port = 25;
            $phpmailer->Username="example1";
            $phpmailer->Password = 'example1pass';

            // Additional settings…
            $phpmailer->SMTPSecure = "tls"; // Choose SSL or TLS, if necessary for your server
            $phpmailer->From = "[email protected]";
            $phpmailer->FromName = "Example One";
            break;

        case 'example2.com':
            $phpmailer->Host="smtp.example2.com";
            $phpmailer->SMTPAuth = true; // Force it to use Username and Password to authenticate
            $phpmailer->Port = 25;
            $phpmailer->Username="example2";
            $phpmailer->Password = 'example2pass';

            // Additional settings…
            $phpmailer->SMTPSecure = "tls"; // Choose SSL or TLS, if necessary for your server
            $phpmailer->From = "[email protected]";
            $phpmailer->FromName = "Example Two";
            break;
    }

}