Why It Matters
Attachments are how business gets done over email — contracts, invoices, reports, design files. But in marketing email, attachments are almost always a bad idea. Spam filters view attachments with suspicion (especially .exe, .zip, or .js files), and large attachments slow down delivery. Plus, most email platforms for bulk sending don't even support them.
How It Works
When you attach a file, your email client encodes it using Base64 and embeds it in the MIME body of the message. This encoding increases the file size by about 33%, so a 10 MB file becomes roughly 13.3 MB in transit. The recipient's client decodes it on the other end.
Gmail caps attachments at 25 MB. Outlook.com allows 20 MB. For anything larger, both platforms automatically suggest cloud links (Google Drive or OneDrive). Most transactional email APIs support attachments but strongly recommend hosting files externally and linking instead.
Quick Tips
- For marketing emails, link to files instead of attaching them — it's better for deliverability and lets you track downloads.
- If you must attach files in transactional emails, stick to PDFs. They're universally readable and less likely to trigger spam filters than Office documents.
- Never send executable files (.exe, .bat, .js) as attachments. Most mail servers will block them outright.