Brevo (formerly Sendinblue) is one of our recommended mailers in WP Mail SMTP. It’s a transactional email provider with scalable price plans, so it’s suitable for any size of business.
I chose Brevo’s because of its free plan that allows to send up to 300 emails a day. Also, they don’t require to enter my credit card to try it out.
If they perform well and my needs require an upgrade, I can always just switch to a higher plan to increase my sending limits.
All you need to do is generate an API Key and connect it with any plugins that requires email such as forms.