How to email a large file (and why it bounced)

Every mail service publishes an attachment limit, and the number you find is almost never the number that stops you. Three things get in the way: the limit counts the whole message, attachments are inflated by about a third in transit, and the ceiling that actually matters belongs to the person receiving it.

The published limits

• Gmail — 25 MB. Google's own help page: "For personal Gmail accounts, the limit is 25 MB."
• Outlook.com — 20 MB. Microsoft's documentation puts the ceiling for internet email accounts such as Outlook.com or Gmail at "20 megabytes (MB)".
• Work and school Exchange accounts — 10 MB by default. Same Microsoft page: "the default email size limit is 10 MB". An administrator can raise it, and many do, but 10 MB is what you should assume about a stranger's corporate address.
• iCloud Mail — 20 MB, with Mail Drop raising it to 5 GB for attachments Apple stores separately for 30 days.

Note that Microsoft's figure for Gmail is lower than Google's. That is not a contradiction — it is the desktop Outlook client applying its own cap to whatever account it is sending through. Whichever piece of software is doing the sending gets a veto.

Why a 24 MB file bounces off a 25 MB limit

Email cannot carry raw binary data, so every attachment is re-encoded into plain text using a scheme called base64. That encoding is about 33% larger than the file it wraps. A 20 MB attachment travels as roughly 27 MB of message.

So the practical rule is to take the published limit and multiply by about 0.7. A 25 MB cap comfortably holds a file of 17 or 18 MB. A 20 MB cap holds about 14 MB. Sizing a file to exactly the limit is how you end up with a bounce message you cannot explain.

That margin is why our presets aim under the line rather than at it: compress for Gmail targets 20 MB, compress for Outlook targets 15 MB, and compress for email targets 5 MB for when you have no idea what is at the other end.

The recipient's limit is the one that decides

Your provider checks its own limit when you press send. The receiving server checks its limit when the message arrives, and if it refuses, the message is already gone from your outbox. This is the single most common reason a file "sent fine" and never turned up.

You cannot look up a stranger's limit, so the useful assumption is the strictest common one: keep attachments under about 10 MB for anyone you do not know, and under 5 MB for a form, a job application or a government address. Small mail servers and older systems are frequently stricter than the big providers, not more generous.

Make the file smaller — what actually works

Different file types have very different amounts of slack.

Photos have the most. A modern phone photo is 3–5 MB and almost all of that is resolution nobody looks at on a screen. Ten holiday photos at 500 KB each still look good in an email and come to 5 MB rather than 40. Compress to 500 KB is a sensible size for anything meant to be looked at rather than printed.

PDFs vary enormously. A text PDF is already small; a scanned one is a stack of photographs in a wrapper and is where the megabytes usually are. 2 MB, 5 MB and 10 MB cover most caps. Our compressor tries a structural re-save first and only rasterises if that misses the target, and it tells you which one it did — worth reading, because rasterising destroys the text layer. Why PDFs get large explains where the weight comes from.

Zipping barely helps. This surprises people. JPGs, PNGs, PDFs, MP4s and Office documents are already compressed, and running a second compressor over compressed data recovers almost nothing — often under 2%. Our own multi-file downloads are written as store-only ZIPs for exactly that reason: the payloads are already JPEGs, so deflating them would cost processing time and save nothing. Zip is useful for grouping thirty files into one attachment. It is not a size fix.

When the file genuinely will not fit

Past a certain point compression is the wrong tool — a 400 MB video is not going to become an attachment, and squeezing it until it does will make it unwatchable. Send a link instead.

Both large providers do this for you. Gmail: "If your total attachment size is greater than the limit, Gmail automatically removes the attachment and adds it as a Google Drive link in the email." Outlook offers the same through OneDrive, and Apple Mail offers Mail Drop, which carries up to 5 GB and expires the file after 30 days.

One thing to check before relying on any of them: the link inherits the sharing permissions of the file. If the folder is restricted, your recipient gets a request-access screen rather than the document. Set the link to work for anyone who has it, or expect a follow-up email.

A few shortcuts that are worth it

• Combine before you compress. Six scanned pages sent as six JPGs are clumsy and larger than one PDF. JPG to PDF builds the document, and merge PDF joins files you already have.
• Convert iPhone photos first. HEIC is smaller but many recipients cannot open it. HEIC to JPG fixes that — though be aware the JPG comes out bigger, because HEIC is the better codec. Compress afterwards, not before.
• Do not compress a document that is about to be read by software. Application portals and applicant tracking systems parse the text layer. There is more on that in resume file size.

All of these run in your browser. Nothing is uploaded, which matters more than usual here — the files people are trying to email are contracts, medical scans and passports.