Base64
A MIME transfer encoding that writes any bytes using 64 safe ASCII characters, four characters for every three bytes; used for attachments and images.
What it means
RFC 2045 defines base64 for email: every 3 bytes become 4 characters from A-Z, a-z, 0-9, + and /, with = as padding, in lines of at most 76 characters. The result survives any mail path, at the cost of about a third more size, plus line breaks.
That overhead adds up. An attachment's encoded size, not its size on disk, is what meets a server's size limit, and images embedded as base64 inside the HTML (data: URLs) inflate the HTML part itself, making Gmail clipping more likely; support for base64 images also varies between mail programs.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 2045, section 6.8: Base64 Content-Transfer-Encodingrfc-editor.org
- Can I email: Base 64 image formatcaniemail.com
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