Custom HTML email templates: which platforms accept them
Most newsletter platforms accept a custom HTML email template, but not in the same way. Mailchimp, MailerLite, Brevo and Buttondown take a whole template, Kit takes one as a wrapper around your writing, beehiiv and MailCheap's editors take HTML only as a block, and Substack has no option for it at all.
A custom HTML email template is a layout you bring from outside your newsletter platform instead of building it in the platform's editor. It might be one you coded, one a designer made for you, or one you downloaded, like the free templates in our library. Whether you can actually use it depends less on the template than on where you send from.
We checked the help pages of eight platforms in November 2026. Here is what each one accepts, what it costs and what it quietly removes.
The short answer, platform by platform
| Platform | Whole template? | Plan needed | What to watch |
|---|---|---|---|
| Kit | Yes, as a wrapper | No plan named in Kit's help | Your writing goes into the template's content slot |
| Mailchimp | Yes | Standard or higher | Legacy builder only |
| MailerLite | Yes | Advanced | Can import a ZIP with images |
| Brevo | Yes | Not stated in Brevo's help | Can be saved as a reusable template |
| Buttondown | Yes | Professional or higher | You edit the full layout HTML |
| MailCheap | Yes, through the API or an AI assistant | Every plan | The editor's HTML block strips styles |
| beehiiv | Only as a block in a post | Paid plans and the free trial | <style> tags are not saved |
| Substack | No | Not available | Rebuild the layout in its editor |
Three ways platforms handle your HTML
The table hides an important difference. Platforms accept HTML in one of three ways, and each changes what your template can do.
The whole email is yours
Mailchimp, MailerLite, Brevo and Buttondown let you paste or upload a complete HTML document and send it as it is. Your <head>, your <style> block, your media queries and your dark mode rules all reach the reader. This is the only route where a carefully built template arrives exactly as it was designed, so it is the one to look for if design matters to you.
The trade-off is that the platform's drag-and-drop tools usually stop working on that email. Mailchimp, for example, only supports custom-coded templates in its legacy builder, and prompts you to switch if you are on the new one. You edit the code, not the blocks.
The template wraps your writing
Kit works differently. A Kit HTML template is a frame: header, footer and styles, with a {{ message_content }} tag where each issue's text goes. You write every issue in Kit's normal editor, and Kit pours it into the frame when it sends.
This suits newsletters that look the same every week, such as a letter or an essay. It suits less well a layout where every issue has different sections, like a links roundup with sponsor slots, because the structure of the content still comes from the editor, not the template.
HTML as a block inside the editor
beehiiv lets you add an HTML Snippet inside a post, and MailCheap's editor has an HTML block. Both are useful for a single element, like a styled call to action or a table, but neither keeps a full template intact.
The reason is what they remove. beehiiv does not save <script> or <style> elements. MailCheap's block removes <style> tags, classes and ids. Any design that relies on a style block (dark mode, mobile layouts, hover states) loses those rules, and only inline styles survive. MailCheap does accept a whole HTML document when you send it through its API or an AI assistant, which puts it in the first group for anyone who sends that way.
Substack sits outside all three. We found no option in its help centre to paste your own HTML email, so the practical route is to use a template as a layout guide and rebuild it in Substack's editor.
What you have to change before pasting
A template that arrives from outside has placeholders the platform does not understand. Four things need attention every time:
- The unsubscribe link. Each platform has its own tag. Kit and Buttondown use
{{ unsubscribe_url }}, Mailchimp uses|UNSUB|, MailerLite uses{$unsubscribe}, Brevo uses{{ unsubscribe }}and MailCheap uses{{unsubscribe_url}}. A template with the wrong tag sends a dead link, and Kit shows an error in the preview when the link is missing or broken. - The postal address. Most platforms add it in their own footer, but when your template replaces the footer, the address has to be in your HTML. Kit offers
{{ address }}for this. - Image addresses. Images must be hosted somewhere public, on the platform or your own server, with full
https://addresses in the code. Relative paths that worked on your computer break in the inbox. - Links. Every link needs to be absolute, starting with
https://.
Our guide to one-click unsubscribe explains why the unsubscribe link matters beyond the legal requirement.
HTML newsletter checks that apply on every platform
Once the template is in, send yourself a test before anyone else sees it. Open it in Gmail and Outlook, on a phone and a desktop. Click the unsubscribe link and confirm it reaches the platform's page.
Then check the weight. Gmail clips HTML over about 102 KB and hides everything below the cut, footer included. A template that is light on its own can cross the line once a long issue is inside it, so measure the final email, not the empty template. More on that in why Gmail clips messages.
Finally, fill in the plain-text version if your platform asks for one. A template built for email usually comes with a text twin; plain text vs HTML emails covers when each version is shown.
Get the setup guide
The table above tells you which platform fits. For the moment you are actually pasting, we made a one-page sheet with the exact menu path in each platform, the unsubscribe tag to use and the before and after checks. Get The Template Setup Guide when you join the list.
Frequently asked questions
Can I use a custom HTML template on a free plan?
On some platforms. MailCheap lists its API on every plan, and Kit's help does not name a plan for HTML templates. Mailchimp needs Standard or higher, MailerLite needs Advanced and Buttondown needs Professional. beehiiv's HTML Snippet is on paid plans and the free trial.
Why does my template look different after I paste it?
Usually because the platform stripped part of it. Editors that take HTML as a block, such as beehiiv's and MailCheap's, remove <style> tags, so anything defined there disappears. Moving those styles inline, onto each element, keeps them.
Will a custom HTML template hurt deliverability?
Not by itself. What matters is the code it produces: a light, valid email with a working unsubscribe link and a plain-text version is no riskier than one built in the editor. Heavy code that gets clipped, or a broken unsubscribe link, can cause problems.
Can I switch platforms and keep the same template?
Yes, if both platforms accept a whole template. You change the unsubscribe tag and the address tag, re-upload the images and send a test. Moving to a platform that only takes HTML blocks means rebuilding the parts that depended on a style block.
Sources
- Mailchimp, Paste in HTML to create a campaign
- Kit, Troubleshooting HTML email templates
- beehiiv, Using HTML in beehiiv posts