• ActiveCampaign – https://www.activecampaign.com – best according to Alicia St. Rose (South Central Coast WordPress Adventure Group)
• MailChimp – https://mailchimp.com
• MailerLite – https://www.mailerlite.com
• ConvertKit – https://convertkit.com
• Featured Images in RSS for Mailchimp & Other Email –
  https://wordpress.org/plugins/featured-images-for-rss-feeds/

• Fluent SMTP Plugin – https://wordpress.org/plugins/fluent-smtp
• SendGrid – https://sendgrid.com