Guide to Handling Email Attachment Image Size Limits: Correct Compression and Conversion Methods

Among globally common email services, Gmail has a total attachment size limit of 25MB per single email. Messages exceeding this limit will be rejected by the system.

Outlook has a total attachment size limit of 20MB per single email, while domestic QQ Mail has a limit of 50MB per single email. All limits apply to the total size of all attachments in a single email.

The first source is total pixel count. The total pixel count of a single photo equals the product of width in pixels and height in pixels. A higher total pixel count results in larger storage space usage.

The second source is encoding format. Different encoding formats have different compression efficiency. At the same visual quality, HEIC and WebP have smaller file sizes than JPG, while uncompressed BMP has a much larger file size than all three formats above. The third source is metadata, which includes shooting parameters, geolocation, and device information. Metadata for some photos can occupy hundreds of KB of space.

Processing image attachments should follow the workflow of resizing first, then adjusting output quality. This workflow allows stable control of the output file size and avoids unnecessary quality loss.

If you adjust quality first then resize, the resizing operation will regenerate pixels and require another volume compression, which will additionally reduce image clarity. Resizing reduces the total pixel count fundamentally, making it the most effective step to lower file size.

In regular email scenarios, recipients only view photos on screens and do not need print-grade large dimensions. Resizing will not noticeably affect viewing experience.

After compressing a 4000-pixel width photo to 1920-pixel width, the total pixel count is reduced by more than 75%, and the file size compression ratio falls within the statistical range shown in the table above.

A 1920-pixel width photo can adapt to most desktop monitors and mobile device screens, meets daily viewing requirements, and a single JPG format photo usually does not exceed 2MB in size.

Photos taken by iPhones are stored in HEIC format by default. This format has higher compression efficiency, but versions of Windows 10 prior to version 1709 do not include native HEIC decoding support, and Windows 7 cannot open HEIC files directly.

If the recipient uses a Windows system, convert HEIC photos to JPG format before sending. You can use a HEIC to JPG converter to complete the format conversion, to avoid the issue where the recipient cannot view the photo.