Do you always need an email marketing service like Mailchimp or Active Campaign to send a HTML email? You can actually use Gmail to send HTML emails as well. You can send these HTML emails as an 1-on-1 message or use a mail merge tool for Gmail like Yet Another Mail Merge to send it to a list of customers or contacts.
Emails sent from Gmail are much more likely to reach primary inboxes than ones sent from an email marketing platform, so if you need to send a message that would benefit from some HTML design (examples: invoices, invitations, quotes, important announcements) to select groups of customers, users or contacts, you can use this method below. (Note: simply copy and pasting a HTML design into Gmail doesn't work.)