If you mail out a newsletter regularly you have more of a chance of retargeting clients that have purchased your products or services before or people who have visited your site and want to keep in touch.
I have used MailChimp for a long time and it is well-loved by my clients. MailChimp is not for people who are technically challenged who want to send out their newsletter themselves.
If you are technically challenged, consider Mailer Lite.