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What email actually needs

Email is the most size-constrained place an image goes. Most providers cap attachments at 20–25 MB, and many corporate mail systems are stricter — some reject anything over 10 MB, and a few over 5 MB. Base64 encoding used by mail transfer adds roughly 33% on top of the file size, so a 20 MB image becomes about 27 MB on the wire.

The practical target is a few hundred kilobytes per image, which keeps a message with several attachments comfortably small and courteous to the recipient’s inbox.

Sensible targets

Resize first, compress second

A modern phone photo is often 4000 pixels wide and 4 MB. Nobody needs that in an email. Resizing to 1600 pixels on the long edge — which still fills a laptop screen — cuts the data by roughly 85% before any compression setting is touched. Doing the resize first means the compressor has far less work to do and the result looks better.

Keeping scans readable

Compressing a scan of a document is the one case where over-compression really bites: JPEG artefacts land on the letter edges and small print becomes smeared. Use PNG or lossless WebP for anything text-heavy, and if it is still too large, reduce the resolution to around 150 DPI rather than crushing the quality.