Compress an image for email on Mac (under 25 MB / 10 MB)

You hit Send and Mail rejects the attachment with “Message exceeds maximum allowed size.” Gmail’s hard limit is 25 MB per message. Outlook is 20 MB. iCloud Mail is 20 MB. Most corporate Exchange servers cap at 10 MB. A single iPhone photo from a recent model can clear all of these on its own.

Why this happens

Email attachments are MIME-encoded, which inflates the byte count by roughly 33% over the raw file size. So a 19 MB photo actually occupies around 25 MB on the wire — which is why Gmail’s 25 MB headline limit is really about 18 MB of actual file. Stack two or three photos and you’re done.

The native way

Three options without installing anything:

• Mail’s built-in resize. When you attach an image in Mail.app, look at the bottom-right of the compose window — there’s an Image Size dropdown. Pick Small, Medium, or Large. Useful but blunt: “Medium” can be 800 KB or 8 MB depending on the source.

• Preview’s export. Open the photo in Preview, File → Export, drag the Quality slider, watch the file size estimate update live. Save the new copy and attach that.

• sips from Terminal. Fast and predictable:

  sips -s format jpeg -s formatOptions 70 -Z 2000 photo.jpg —out smaller.jpg

  The -Z 2000 caps the longest edge at 2000px. A 12 MB iPhone photo drops to around 800 KB-1.2 MB.

The faster way

For multiple images, or when you want it done in one drag, Download Melt. It’s a $9.99 Mac app that compresses and resizes in a single pass.

1. Download Melt and open it.
2. Drag the images in — one or a hundred.
3. Set quality (75-80 is fine for email), and optionally cap the longest edge at 2000-2400px.
4. Click Compress and attach the output files.

Melt strips EXIF too, so location data baked into your photos doesn’t ride along to the recipient — which honestly matters more than most people realise.

How much smaller will it really get?

A typical 48 MP iPhone photo (~5 MB on-device, but expanded to ~12 MB when AirDropped or exported) lands at 600 KB-1 MB at quality 80 with a 2000px cap. A burst of ten photos goes from 50+ MB to 6-8 MB total — comfortably under every mainstream email limit.

If the recipient genuinely needs full-resolution files, share via iCloud Mail Drop (built into Mail, supports up to 5 GB) or a link to a Drive/Dropbox folder. Email attachments shouldn’t be doing that job.