Outlook's attachment ceiling is not one number, and picking the wrong one is how messages bounce. Microsoft's own support pages state three: “For internet email accounts such as Outlook.com or Gmail, the email size limit is 20 megabytes (MB)”; “The attachment size limit for files is 25 MB” for Outlook.com; and “For Exchange accounts (business email), the default email size limit is 10 MB” — with Outlook Web App notes putting the default whole-message limit at 25 MB or 35 MB depending on the hosting service, and warning that the real figure is set by the organisation. All of those limits count the whole message, text and images together, and Microsoft says it plainly: “Resizing an image doesn't reduce its file size.” Changing dimensions in an image editor or in Outlook itself does nothing for the number that matters. You have to re-encode. A 12-image newsletter at 900 KB per image is 10.8 MB before a single word of text, which is already over the Exchange default. Compressing those images to 150 KB each puts the same campaign at 1.8 MB with no visible difference at the width an email renders. Skwish resizes and re-encodes in the browser, so a work machine never uploads client material to a third-party server.