WordPress SMTP plugins, there are a few that I've found reliable. One of my favorites is Easy WP SMTP. It's super easy to set up and works like a charm. Another good one is WP Mail SMTP. It's got a lot of great features and is user-friendly.
If you're looking for something with a bit more customization options, you might want to check out Post SMTP Mailer/Email Log. It's got a ton of advanced settings for fine-tuning your email delivery.
I'd recommend giving them a try and seeing which one works best for you. And if you need more info, www.frogstonemedia.com has some great articles on WordPress plugins that might help!