constantcontact.com
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Constant Contact is an email marketing platform for small businesses that lets you build and send campaigns with templates, automation, sign-up forms, landing pages, and analytics; the site also offers tutorials, guides, and resources on email marketing best practices.
More resources on Email Marketing
mailchimp.com
Mailchimp is an all-in-one marketing platform focused on email marketing, automation, and audience management. It offers email templates, automation workflows, landing pages, CRM features, and a wide range of tutorials, guides, and resources to help you plan and optimize campaigns.
hubspot.com
HubSpot is a marketing, sales, and CRM platform whose site offers extensive educational content on email marketing. It features blogs, how-to guides, tutorials, templates, and free tools to plan, create, automate, and optimize email campaigns.