What is a base64-encoded body?

A base64 encoded body transforms binary content into ASCII safe text so it can travel through SMTP, which only supports seven bit characters.

This encoding expands size by roughly one third because base64 turns three bytes into four characters. This is why attachment limits are tighter than expected. A twenty megabyte attachment often exceeds a twenty five megabyte cap once encoded.

Most clients decode base64 automatically and reveal the content without the reader noticing the conversion.