Tools:

• https://mjml.io/
• https://get.foundation/emails/getting-started.html
• https://maizzle.com/
• https://heml.io/
• https://bojler.slicejack.com/

MailChimp Helpers:

• https://mailchimp.com/help/create-editable-content-areas-with-mailchimps-template-language/
• https://mailchimp.com/help/templates/

Testing:

• SendInBlue (limited 50 testing mails to test / daily)
• HTML quick check: https://www.htmlemailcheck.com/check/
• CSS SUPPORT: https://www.campaignmonitor.com/css/

Important to test:

• Gmail
• Outlook dont love etc.. prefer
• Roundcube

Dont forget for custom emails:

• DKIM and SPF records checker
• SPAM tester: https://www.mail-tester.com/
• Deliverability tester: https://www.sendforensics.com/email-deliverability-test

Affordable:

• https://emailpreviewservices.com/en/pricing
• https://www.getresponse.com/?version=platform_1

ALL:

• https://www.mailpoet.com/blog/email-testing-tools/