Why email blocks HTML attachments
An HTML file is a small web page, and a web page can run scripts and show a convincing sign-in form. That makes HTML attachments a favourite of phishing: a fake Microsoft 365 sign-in page, sent as an attachment, never touches a website a filter can block. So mail systems treat them with suspicion.
What happens depends on who's receiving it. Many company mail systems quarantine or strip .html and .htm attachments outright, under policies set by their IT team. Others let them through with a warning. Some mail apps show the file only as code, or refuse to open it. You often can't tell which happened: the email arrives, and the attachment doesn't.
AI tools make this more common. Claude, ChatGPT and Gemini make their best work as HTML, because it can hold flow charts, animation and slides in one light file, so more people now have an HTML file they want to send.
The usual workarounds, and their catches
| Workaround | The catch |
|---|---|
| Zip it | Many filters look inside zip files, and some block zips too. The reader has to unzip and open a file their IT team warned them about. |
| Rename it to .txt | The reader sees code, and has to rename it back. It looks exactly like what phishing does. |
| Convert it to PDF | Animation, slides, interactive charts and anything that opens on a click are lost, and the page breaks fall where they fall. |
| Put it in a shared drive | Most drives show an HTML file as code rather than run it, or ask the reader to download it, which lands it back in the same warning. |
| Host it on a website | Anyone with the link can read it, you can't tell who did, and nobody can comment on it. |
| Share it from your AI tool | Works well if your reader uses the same tool. Otherwise they may need an account with it. See sharing AI artefacts with people who don't use your AI. |
Sharing it through aihops
aihops was built for exactly this. Instead of the file, your reader gets an email with a link, and nothing to block.
- Put the page on aihops. Upload the HTML file, paste the HTML, ask Claude or ChatGPT to share it, or email it as an attachment to share@aihops.com.au (aihops's own mailbox accepts it, and only from a sender whose email checks out).
- Add the people who should read it. Up to 50 at once, by email address. Only they can open it.
- They sign in and read it. With a code sent to their email, or Google. The page works as it did when it was made, scripts, slides and all, in a sandbox walled off from aihops and from their session.
- See who read it, and what they said. You see who opened it and for how long. Collaborate, and they can comment and suggest edits pinned to the words, and AI can make the next version from everyone's mark-up.
Sharing is free, with unlimited documents and people. We don't use your content to train AI models, and we don't sell it.
If you do email it
- Tell the reader it's coming, and what it is, in the body of the email, so a quarantined attachment gets noticed.
- Check it opens offline: an HTML file that loads scripts or fonts from the web may look broken when opened from an inbox.
- Never ask anyone to turn off a warning to open it. If their mail system blocks it, send a link instead.