If you need good SMTP plugins for WordPress, check out WP Mail SMTP, Easy WP SMTP, and Post SMTP Mailer/Email Log. They're solid choices for managing email delivery from your site.
It somewhat depends on your scenario. Do you want to force every outbound mail through a remote SMTP server? Because that can be confiured serverwide. If you are merely looking to integrate a number of sites with a transactional mailer, WP Mail SMTP is your best bet. However, if you happen to use Gravity Forms, you might want to look at Gravity SMTP.