Email attachment size limits, and how to fit under them

“Attachment size exceeds the allowable limit.” It is one of the few error messages that is both completely clear and completely unhelpful, because it does not tell you what the limit is or what to do about it.

The actual limits

Service	Sending limit	Receiving limit
Gmail	25 MB	50 MB
Outlook.com	20 MB	20 MB
Microsoft 365 (business)	25 MB, admin-configurable to 150 MB	same
Yahoo Mail	25 MB	25 MB
iCloud Mail	20 MB	20 MB
Proton Mail	25 MB	25 MB
Zoho Mail	20 MB (free)	20 MB

Two things are worth extracting from that table.

First, the receiving limit is what actually matters, and it is not yours to control. Gmail will happily let you send 25 MB, but if your recipient is on a corporate Exchange server configured to 10 MB, the message bounces — sometimes silently, sometimes hours later.

Second, the limit is on the whole message, not each attachment. Five 5 MB files is 25 MB before you have written a word.

The 33% nobody mentions

Here is the part that catches people out. Email attachments are encoded in Base64, which represents three bytes of binary data as four ASCII characters. That is a 33% size increase, plus a small amount of line-break overhead.

So a 20 MB attachment becomes roughly 27 MB on the wire, and Gmail’s “25 MB limit” is applied to the encoded size. In practice this means your real ceiling for attachments in Gmail is about 18–19 MB of actual file, not 25.

If you are close to a limit and it keeps failing for no apparent reason, this is usually why.

Getting under the line

Photographs

This is the common case and the easiest to fix. A modern phone photo is 3–6 MB, and almost all of that is unnecessary for anything that will be looked at on a screen.

Run them through the compressor with a target of 300–500 KB each. At that size a photograph is still perfectly sharp on any monitor, and ten of them fit comfortably in a 5 MB message where before they were 40 MB.

If the recipient only needs to look at them rather than print them, resize to 1600 pixels on the long edge first. Combined with compression, that typically gets a phone photo under 200 KB with no visible loss on screen.

Scanned documents

Scanners default to settings far beyond what a document needs. A text page scanned at 600 dpi in full colour can easily be 15 MB; the same page at 200 dpi greyscale is under 500 KB and just as readable.

If you already have the large file, render the PDF to images at 144 dpi, compress them, and rebuild the PDF. It is a few more clicks than it should be, and it works.

Many files

Two separate problems get solved differently.

If they are documents, merge them into one PDF. This does not reduce total size much — a merged PDF is roughly the sum of its parts, minus deduplicated fonts — but it turns twelve attachments into one, which is easier for the recipient and avoids per-attachment count limits.

If they are photos, compress them individually first. Zipping already-compressed images saves almost nothing: JPEG, PNG, WebP and HEIC are all already compressed, and ZIP has nothing left to find. A zip of ten JPEGs is about the size of ten JPEGs.

Zipping still helps for text files, spreadsheets, CSVs and source code, where 70–90% reductions are normal.

When to stop fighting the limit

Sometimes the file is simply large — a video, a design file, a 400-page scanned archive. At that point the right answer is not compression but a link.

Every major mail provider has this built in. Gmail switches to a Google Drive link above 25 MB; Outlook offers OneDrive; iCloud Mail Drop handles up to 5 GB and expires after 30 days.

The trade-off is worth being conscious of: a link means the file lives in cloud storage with whatever sharing permissions you set, often “anyone with the link”. For a photograph of a passport being sent to a lawyer, that is a meaningfully different privacy posture from an attachment sitting in two mailboxes. Set an expiry and restrict access if the provider allows it.

A note on what compression cannot do

If someone tells you a file “must be under 5 MB” and it is a 300-page scanned document, no compression setting will get you there while keeping it readable. The honest answer is to split it, send it in parts, or use a link.

Tools that promise otherwise are usually about to hand you something illegible.