What are the email attachment limits?

Gmail accepts 25 MB per message and switches to a Drive link above that. Outlook.com and most Exchange defaults sit at 20 MB, and many corporate gateways are configured lower. Base64 encoding inflates an attachment roughly 33% on the wire, so a 20 MB file can be rejected by a 25 MB limit.

What makes a self-contained HTML report so large?

Three things, in order: inlined JavaScript libraries (plotly.js alone is about 3 MB per figure if inlined repeatedly), base64 images and screenshots, and raw data embedded for client-side filtering. Test reports add videos and traces on top.

How do you shrink one?

Inline the chart library once rather than per figure, switch screenshots to `retain-on-failure`, downsample the embedded dataset to what the charts actually plot, and gzip the file if the recipient can handle a `.gz`. Those four typically cut an order of magnitude.

It helps on the wire and hurts on arrival — many gateways quarantine or strip zips containing HTML, and the recipient still ends up opening the file from `file://`, where XHR-loading reports break. It solves the size symptom and adds two new failure modes.

What is the alternative to attaching it?

Send a link. Publishing the report puts it behind a URL with no attachment cap, it renders in the browser rather than being downloaded, and the reply happens as a comment on the relevant paragraph instead of a reply-all thread.

The report bounced: attachment too large

Message exceeds the maximum size — or worse, no bounce at all and a recipient who never mentions it.

The limits you are hitting

Where	Cap
Gmail	25 MB (auto-switches to a Drive link above it)
Outlook.com / Exchange default	20 MB
Many corporate gateways	10 MB, or lower
iCloud Mail	20 MB

And a detail that catches people: attachments are base64-encoded in transit, which inflates them by about a third. A 20 MB file is roughly 27 MB on the wire — over Gmail's cap despite looking under it.

What is making it big

In descending order of blame:

Inlined chart libraries, repeatedly. plotly.js is around 3 MB. Export ten figures with include_plotlyjs=True on each and you have inlined it ten times. Inline once:

parts = [fig.to_html(full_html=False, include_plotlyjs=(i == 0))
         for i, fig in enumerate(figs)]

Base64 media. Screenshots, videos and traces in test reports. In Playwright, screenshot: 'only-on-failure' and video: 'retain-on-failure' usually remove most of the payload with no loss to anyone reading a green run.

The whole dataset, embedded. Reports that support client-side filtering ship the raw rows. Aggregate to what the charts actually plot, or cap the table and link the full extract.

Font files, inlined. Rarely worth their size in a report nobody prints.

gzip -9 report.html will often cut what remains by 80% — but a recipient who has to un-gzip is a recipient who might not.

Why zipping is a bad answer

It is the reflex, and it introduces two problems:

Corporate mail gateways commonly quarantine or strip archives that contain HTML, because that combination is a phishing pattern. The message arrives with the attachment removed and no explanation.
The recipient unzips and double-clicks index.html, landing on file:// — where XHR-loading reports render blank (Allure, Playwright) and relative asset paths break (lost CSS).

Email clients also strip <script> and external stylesheets from inline HTML bodies, and Outlook on Windows renders with the Word engine — no flexbox, no grid. Details in emailing an HTML report.

Send a link instead

There is no attachment cap on a URL. Publish the report, paste the address:

It opens in the browser, on a phone, in a Slack unfurl.
Interactivity survives — scripts run in a sandboxed iframe (allow-scripts, no allow-same-origin).
Access is per-report: private, invited people only, anyone with the link, or public.
Replies land as comments on the paragraph in question, instead of a reply-all thread where nobody can tell which chart "this number looks wrong" refers to (comment on HTML).

Limits: entry HTML 5 MB; assets 25 MB per file, 250 MB and 500 files per report — so the multi-megabyte media that broke your mail server uploads as assets rather than being inlined into the document.

Try it

Comma is free — unlimited reports, unlimited commenters, unlimited revision history.

Publish the report and send a link →