Why do email attachments have size limits?

Answer: They keep mail servers from getting overloaded by very large files.

They keep mail servers from getting overloaded by very large files. ✓ — Correct! Email servers handle millions of messages, so a very large attachment consumes bandwidth and slows delivery for everyone. Limits such as 25 MB at Gmail and 20 MB at Outlook keep one message from becoming a bottleneck. The original email protocol, SMTP, was not designed for huge files, so cloud storage links are often used instead.

They keep old messages from filling up mail servers' storage space. — This gets part of the picture: storage is a real cost for mail providers. But the attachment limit is mainly about one message's file size straining delivery, not old messages piling up. A huge file still has to be handled as part of the message before the recipient can download it.

They stop unauthorized people from opening the files you attach. — Privacy matters to mail providers, but attachment limits are not a security switch that keeps the wrong people out. Access depends on your account and the recipient's, not on file size. The limit addresses how much data the mail system must carry, not who may open it.