Does Outlook Compress Images? Inline Yes, Attached No

Attached, no. Inline, yes, and more aggressively than most people expect. Does Outlook compress images? On 2026-07-18 I ran a 4.13 MB iPhone photo through both paths. As a file attachment it arrived byte for byte identical, same sha256 checksum. Inserted inline in the message body it came back resized from 4032x3024 to 1920x1440, cut to 819.1 KB, 80.6 percent smaller, with its EXIF metadata stripped out.

A word on scope before anything else. These four numbers are what Outlook did to two reference files, on one day, through two named paths. They are not a claim about every build, every account type, or every insertion method, and the version differences further down this page come from Microsoft documentation rather than from my test bench. Measured and documented stay labeled as what they are.

Does Outlook Compress Images? The Measured Verdict

Split verdict: the attach path is a vault and the inline path is a wood chipper. The same two files went through both paths on the same day, and only one path gave them back.

The reference files this whole series uses

REF-A is a real iPhone photo: 4,332,705 bytes, 4032x3024 pixels, JPEG, carrying the full EXIF block a phone writes, meaning the hidden metadata that records camera model, timestamp, and GPS coordinates. REF-B is a photographic cutout with genuine transparency: 1,863,465 bytes, 1240x930, PNG with an alpha channel. Every platform in this project meets the same two files, so any page's numbers line up against any other's. The full scoreboard lives on the hub that collects every app compression test we have run.

What the attach path returned

Both attached files came back untouched. Not visually similar, not close in size: the same bytes, confirmed by matching sha256 checksums on the delivered copies. Two files with the same sha256 fingerprint are the same file. An encoder cannot touch a single pixel without changing that fingerprint, so the attach path in my test provably applied no compression at all.

What the inline path returned

The inline JPEG came back a different object entirely: 819.1 KB against the original 4.13 MB, resized from 4032x3024 down to 1920x1440, EXIF block gone. The inline PNG kept its 1240x930 pixels and its PNG format but still shed 16.8 percent of its bytes to a re-encode, landing at 1.48 MB.

What one test can and cannot prove

It proves the attach path shipped originals and the inline path rewrote both files, on the path I ran. It cannot tell you that every Outlook surface behaves identically, and I did not measure the mobile apps or every insertion method. One dated data point with checksums still beats a decade of forum guesswork, which is most of what this question had.

Does Outlook Compress Photos You Attach?

No. Attached photos crossed intact in my test, down to the last byte, and that carries a useful side effect: everything inside the file survived too.

The checksum is the whole argument

I hashed the files I sent and the files that arrived, and both pairs matched. A sha256 match is binary proof of non-interference; no compressor, optimizer, or resizer can leave a hash standing. Whatever Outlook does elsewhere, the attach path treated my photos as sealed cargo.

EXIF rides along, for better or worse

Identical bytes mean the photo's EXIF metadata arrived complete, GPS coordinates included. That is excellent for archiving originals and worth pausing on for privacy: an attached photo quietly tells the recipient where it was taken. If that should not happen, strip the metadata before the file ever reaches Outlook. Our compressor removes it by default, no setting to find.

The one attachment setting worth knowing

Classic Outlook keeps a per-message switch under File, then Info, labeled Image Attachments, where you choose between resizing large attached images on send and leaving them alone. Microsoft's picture compression documentation walks the exact path. The choice applies to that one message only. On the path I measured, the delivered files were untouched originals.

The ceiling is about size, not quality

Microsoft's attachment size guidance puts the default limit at 20 MB for internet email accounts and 10 MB on default Exchange setups. Email encoding inflates every attachment by roughly a third in transit, so a 20 MB account realistically carries about 15 MB of files per message. Past the ceiling, Outlook steers you to a OneDrive link, which hands over the untouched original by a different road. The limit blocks a send; it never quietly compresses one.

What Happens to Images Inserted Inline in Outlook?

Inline images get rebuilt in transit. My inline JPEG lost 80.6 percent of its bytes, dropped to 1920x1440, and lost its metadata. The PNG kept its pixels and still came back 16.8 percent lighter. Here are the four results side by side, then the folklore they overturn.

The 1,672 pixel claim did not survive contact

A long-running Microsoft Learn thread on Outlook resampling pins the inline resize threshold at roughly 1,672 pixels, a community observation repeated for years. My delivered inline JPEG measures 1920 pixels on the long side, above that supposed ceiling. Either the threshold has changed, or it never applied to the path I used. I can only vouch for my own tape measure, and mine reads 1920.

The PNG result quietly kills another myth

Forum wisdom says Outlook turns PNGs into JPEGs. On a single send through my path, the PNG stayed a PNG at 1240x930 and lost 16.8 percent of its bytes to re-encoding, no format flip. The conversion reports mostly come from long reply and forward chains, which I did not simulate. My one-send verdict: same format, same pixels, fewer bytes.

The fix is choosing the input, not fighting the pipeline

An inline image is a layout decision, and you can make it with a file you already control. Compress the photo to the size you actually want before inserting it, and the re-encode has nothing meaningful left to take. An input you sized on purpose beats an output you did not choose.

Why Does Outlook Recompress Inline Images?

Because classic Outlook does not treat an inline image as a file. It treats it as part of a document, and the document engine is Word: the classic Windows client composes every message in Word's renderer, and Word has strong opinions about images.

Word thinks in dpi, and it wants 96

When you insert an image, the engine reads the dpi value stamped in the file's metadata and rescales the display to normalize it to 96 dpi. Robert365's breakdown of unsharp Outlook images shows the arithmetic: an image tagged 150 dpi at 88 pixels tall renders at 56 pixels, because 88 divided by 150 times 96 is 56.32. On send, the engine re-encodes the image at its new dimensions, and that re-encode is permanent.

The default resolution dial sits at 220 ppi

The same Editor Options panel that hides the off switch also carries a Default resolution setting, parked at 220 ppi unless you raise it to High fidelity. That default is tuned for small messages, not for photographs, and it applies to everything inline. High fidelity is not universal either: it arrived in Microsoft 365 builds around version 2109, and volume-licensed Outlook 2019 never got it.

Every reply is another trip through the chipper

JPEG is a lossy format, so each re-encode throws away detail that never comes back, and a busy thread re-renders its images on every send. Two forwards in, an inline photo has been through the pipeline three times. The mechanics of why generational loss compounds live in our guide to lossy versus lossless compression. The practical rule: anything that must survive a thread travels as an attachment, not as decoration.

Does Every Outlook Version Compress Images the Same Way?

No, and this is where the question splits into four answers. Ask does Outlook compress images and the honest counter-question is: which Outlook? Four surfaces share the name and almost nothing else.

New Outlook took the dials away

New Outlook for Windows is a packaged web app, not the Word-based client, and the Image Size and Quality panel does not exist in it. Microsoft's own guidance states you can change the display size of an inserted image but cannot reduce its file size. If your compression toggles vanished after switching clients, they did not move; they are gone. Outlook for Mac is the surface nobody documents well; my sources are silent on it and I have not tested it, so this page makes no claim about it.

Outlook mobile asks, once, past 1 MB

On iOS and Android, Microsoft documents a prompt that appears when attached images push a message past 1 MB combined, offering resize or keep original sizes, with the answer applied to every image in that draft. There is no granular quality control. I have not measured the phone apps, so treat this as documented behavior rather than verified numbers.

How to Turn Off Image Compression in Outlook

Two switches, both in classic Outlook only, one per message and one global. Set both when quality matters, then remember the pipeline still exists underneath them.

Switch one: protect attached images

1. Attach your images to the message in classic Outlook.
2. Click File, then Info.
3. Under Image Attachments, select Do not resize images.

The choice is per message and covers attachments only. It does nothing for images sitting inline in the body, which is the single most repeated misunderstanding in the support threads.

Switch two: protect inline images

1. Click File, then Options, then Mail, and open Editor Options.
2. Go to Advanced and find Image Size and Quality.
3. Check Do not compress images in file.
4. Set Default resolution to High fidelity.

Two caveats, both earned. The panel exists only in Microsoft 365 builds from around version 2109 onward, so Outlook 2019 volume license installs will not find it. And users in the same Microsoft Learn thread that tracked the resampling threshold report the setting reverting on its own, so verify it before a send that matters.

When the toggles are not enough

The reliable moves live outside the settings panel. Send the file as a true attachment, hand over a OneDrive link so the original never touches the message body, or shrink the image deliberately before it goes in. Format choice helps at the margin: photographs travel best as JPEG, while screenshots and diagrams hold their edges as PNG, and the community recipe of saving PNGs at exactly 96 dpi to slip through untouched deserves suspicion, since my reference PNG was re-encoded anyway. One more compatibility note: iPhones shoot HEIC by default and plenty of clients still choke on it, so convert HEIC to JPG before a photo goes anywhere near an email. The full workflow, including what other providers do to your files, is in our guide to compressing images for email.

How Does Outlook Compare to the Other Platforms I Tested?

Outlook's inline pipeline dealt the roughest treatment of the two mail clients, but it is a gentle touch next to what the social feeds did. Every platform below received the identical 4.13 MB reference photo.

The two mail clients split on philosophy: Gmail recompressed its inline copy without touching a pixel, while Outlook resized, recompressed, and stripped metadata in one pass. The feeds went further: Facebook cut the same photo to 579.7 KB at 2048x1536, and LinkedIn went all the way down to a 22 KB thumbnail. Meanwhile the attach paths on both mail clients, and even WhatsApp's document send, delivered checksum-identical files. The pattern across the whole test set is consistent: platforms preserve what they treat as files and rebuild what they treat as content.

Pick the Path Before Outlook Picks for You

So does Outlook compress images? Not the ones you attach: mine arrived hash-identical. The ones you place inline get resized, re-encoded, and stripped of metadata, at least on the path I measured, and the settings that promise to stop it are classic-only and known to wander. The dependable answer is to control the file before Outlook ever sees it.

When the image needs to be smaller anyway, size it down on your own terms first: quality 80 by default, which cuts most photos roughly in half before anything visible changes, EXIF stripped automatically so an emailed photo does not carry your location, and every upload deleted within the hour. Then attach the result, and let Outlook be a courier instead of an editor.