When it comes to getting your emails delivered to the inbox – and not spam – your content and setup matter. But there’s one often-overlooked factor that can seriously impact deliverability: attachments.
Whether you’re sending a whitepaper, a PDF invoice, or a proposal, how you handle attachments can make the difference between inbox or junk folder.
🧠 Why Attachments Can Trigger Spam Filters
Spam filters treat attachments with extra cauciness – and for good reason. Malicious actors frequently use attachments to spread viruses, phishing attempts, or malware. This is why filters have strict rules around it, specifically:
- File type
- File size
- Number of attachments
If your attachment raises red flags, even a technically perfect email can be flagged as suspicious.
🚫 Risky Attachment Types to Avoid
Some file types are almost always considered dangerous:
- .exe, .bat, .cmd, .js, .jar, .vbs, .scr
- .zip or .rar (especially if they contain executable files)
- .html files (can contain scripts or phishing forms)
Even if your intentions are legitimate, many filters auto-block or quarantine emails with these files.
✅ Safer formats include:
- .docx, .xlsx (though Microsoft files can also be flagged if macros are enabled)
- .txt
- .jpg, .png, .gif (standard image formats)
📏 Attachment Size: What’s Too Big?
Large attachments can also hurt deliverability – or block delivery entirely.
Most ISPs and corporate servers have file size limits:
- Gmail: 25 MB total (attachments + message)
- Outlook: 20-25 MB
- Yahoo: 25 MB
- Zoho, Proton, others: often 15-20 MB
- Corporate mail servers: can be as low as 10 MB
👉 Best practice: Keep attachments under 10 MB, and ideally under 5 MB for wide compatibility and better inboxing.
🧩 Deliverability Impact: What Happens If You Ignore the Limits?
If your email contains unsupported or oversized attachments:
- 📉 Your open rates drop (email ends up in spam or not delivered at all)
- 🚫 It can be rejected by the receiving server
- 🛑 You might get blocked or rate-limited by the ISP
- 🔁 Bounce rates increase, hurting your sender reputation
💡 Best Practices for Sharing files and Sending Attachments
If you absolutely must include an attachment, follow these guidelines:
✅ Try hosting your files on a reliable cloud service and provide the client with a secure link to access the file.
Avoid using free file-sharing platforms, as they may be associated with poor sending reputations and could harm your deliverability.
Make sure the file is protected under the client’s account or access settings, for example, by requiring login or restricting permissions.
📎 Include the link with a clear call to action, such as:
“Download the report here” or “View the full proposal online”
🔐 Bonus: You can track clicks and engagement with link analytics – something traditional attachments can’t do.
✅ Use descriptive filenames (avoid generic or suspicious names)
✅ Add real HTML content to the body – don’t just send an empty message with a file
✅ Include a sentence explaining the attachment:
“Attached is our Q2 proposal in PDF format for your review.”
P.S. Keep in mind that attachments increase the size of your email traffic, which can significantly slow down sending speed – especially when your MTA is under heavy load.
Better yet?
📬 Final Thought
Attachments aren’t bad by default, but they are high risk for deliverability if not used correctly.
Ask yourself:
- Can this be a link instead of an attachment?
- Is the file type clean and safe?
- Is the size optimized?
- Will it pass through corporate firewalls?
When in doubt, lean toward links, clean formatting, and lightweight delivery. Your deliverability and your audience will thank you.