Email attachments and compressed folders

An email attachment is a file that travels with a message. Email services set a maximum total size for the attachments in one message, so a single large file, or a set of files, can be rejected and never leave your outbox. Compression rewrites a file in a more compact form, which can bring it back under the limit so that the message sends.

One file instead of many

Compression can squeeze a set of files into a single smaller file, and that one file is what you attach. Everything stays inside it, so the recipient collects the whole set in one download instead of picking the pieces out of several messages, and nothing gets left behind. When they open it, their computer decompresses it and the original files appear again.

• There is one attachment to download rather than several separate ones, so it arrives sooner.
• The files stay together as a set, in the order and folders you chose.

A worked example

The eco club's greenhouse sensor logs the temperature every 30 seconds, and after a term its log file is 62 MB. The club's email service allows 20 MB of attachments per message, so the file is refused. Compressing it brings it down to about 8 MB, well under the limit, and the researcher studying the school's local climate receives every reading.

A set of files like this is compressed with lossless compression, so once it is decompressed every file is exactly as it was. That matters for measurements, documents and program files: lossy compression would delete data for good, and a damaged program file would not run.