What is a base64-encoded body?
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A base64-encoded body is what happens behind the scenes when an email contains binary data (like images, PDFs, or any attachment). SMTP, the protocol that delivers email, only speaks 7-bit ASCII. It can't carry raw binary data without corruption. So before your attachment travels, it gets converted into plain text characters using base64 encoding. The receiving server decodes it back to the original file.
This conversion happens automatically. You never see it unless you peek at the raw email source. Most email clients decode base64 instantly and show you the attachment as if nothing happened.
The catch: base64 encoding inflates file size by about 33%. Three bytes of binary data become four characters of text. That's why a 20 MB attachment often exceeds a 25 MB sending limit once encoded. If you've ever hit a size limit with an attachment that looked smaller than the cap, this is why.
This matters most when you're sending large files or embedded images. If your newsletter includes high-resolution photos embedded as base64, the email size balloons fast. That can slow delivery, increase the chance of clipping in Gmail (which truncates messages over 102 KB), or push you over your ESP's size limit entirely. Some senders avoid this by hosting images externally and linking to them instead of embedding. That keeps the email lean, but it means images won't display if the recipient blocks remote content. Trade-off: smaller email size vs. guaranteed image display.
One more thing: base64 encoding applies to the email body when it contains non-ASCII characters (like emoji or non-Latin scripts). If your subject line or body uses anything beyond basic English letters and numbers, it might get base64-encoded too. Most modern email systems handle this without issue, but older systems sometimes struggle with encoded subject lines.
If you're troubleshooting delivery issues with attachments or images, check the encoded size. Our Source Analyzer shows you the raw MIME structure, including base64 sections and their actual size. Or just ask us if something feels off (we're here for that).
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