Understand the result
Why a small attachment grows
3 raw bytes → 4 base64 characters 4 / 3 − 1 ≈ 33.3% encoding overhead
Line wrapping, MIME boundaries and headers can add more bytes beyond the base64 characters. The inspector distinguishes decoded base64 content from serialized message size; neither number proves that a provider will accept a message.
Inspect a representative message
Use a synthetic sample first. Then load a supported local file or paste the raw source. Compare normalized message size with encoded attachment bytes and review warnings. A size diagnostic does not establish sender authenticity or whether an attachment is safe.
What this tool can and cannot tell you
Base64 expands binary data to roughly four bytes for every three input bytes, before line breaks and MIME headers. Provider message limits apply to encoded messages. This diagnostic does not send email or test spam placement.
Common questions
Why is my attachment bigger after encoding?
Base64 expands groups of three bytes into four characters and may add padding. MIME framing and line wrapping add more bytes.
Will this send the email or upload the .eml file?
No. File reading and parsing occur in the browser workspace. The tool has no send-email operation.
Can I share the report without exposing recipients?
The report deliberately omits raw content and identifying headers. Review the resulting file before sharing; do not share your original message by mistake.
Worked examples are synthetic. No customer outcome or saving is implied. Content by Orr Yoffe.