⚖️ The Core Differences: Embedded vs. Hosted
Factor | Embedded Images (CID/Base64) | Hosted Images (External URLs) |
Visibility in Clients | Immediate in most clients (bypasses security blocks). | Blocked by default in clients like Outlook and Thunderbird until manually downloaded. |
Email Size | Larger because the image file is attached directly to the message payload. | Smaller because the email only contains a lightweight text link to the image. |
Attachment Indicator | Often triggers a paperclip/file attachment icon in the recipient's inbox. | Clean inbox appearance with no attachment icons or clutter. |
Spam Filtering | Slightly higher risk of triggering strict corporate spam filters due to the attachment payload. | Safer and more deliverable (when served over secure HTTPS links). |
Appearance in Threads | Often dropped or stripped during replies and forwards. | Usually retained throughout long threads via HTML quoting. |
Editing Post-Send | Cannot be updated. Once sent, the recipient has that specific file forever. | Can be updated instantly on our servers, updating the image in previously sent emails. |
Open Tracking | Not possible to track image-based opens. | Possible using hosted analytics. |
📱 How Different Email Clients Handle Your Images
Because every email client (Outlook, Gmail, Apple Mail, etc.) has its own rules, your signature images may behave differently depending on who you are emailing. Refer to the table below to see how major clients treat each type:
Email Client | Embedded Images (CID/Base64) | Hosted Images (External URLs) |
Outlook (Windows) | Shows inline; marked as an attachment; stripped from replies. | Blocked by default; requires the user to click to download. |
Outlook.com / OWA | Displays inline; treated as an attachment. | Blocked unless the sender is marked as trusted. |
Outlook Mobile | Displays inline; cannot add via mobile signature. | Usually auto-loaded. |
Apple Mail (macOS) | Displays inline; stripped or resized in replies. | Auto-loaded via Mail Privacy proxy. |
iOS Mail (iPhone/iPad) | Displays inline; removed entirely from replies. | Auto-loaded by default. |
Gmail (Web & Mobile) | Displays inline; exceptionally large images may show as attachments. | Auto-loaded via Google's secure proxy. |
Yahoo Mail (Web) | Displays inline. | Auto-loaded unless the email is marked as Spam. |
Thunderbird | Displays inline; marked as an attachment. | Blocked by default. |
⚠️ Important Considerations Before Choosing
1. The Reply & Forward Limitation (Especially iOS & Outlook)
The biggest tradeoff with embedded images is what happens during a back-and-forth email conversation. Major clients like iOS Mail and Outlook for Windows automatically strip embedded attachments when a user hits "Reply" or "Forward".
The Result: Your recipient will see empty boxes, alt-text, or placeholders like
[image]instead of your logo in subsequent replies.If your team communicates frequently on long email threads with mobile/Outlook users, Hosted Images are highly recommended to keep your signature intact.
2. WiseStamp Deployment Compatibility
Embedded images are not supported by all installation methods. To use embedded images, you must use a deployment method that injects the signature at the server or system level:
Supported Deployments (Embedded):
✓ Server-Side Injection (Google Workspace & Microsoft 365)
✓ Outlook Add-in
Unsupported Deployments (Falls back to Hosted):
✗ WiseStamp Chrome Extension
✗ WiseStamp Desktop App
💬 Frequently Asked Questions
Why can't I see my signature preview while composing an email?
If your team uses Server-Side Injection (which is required for embedded images), your signature is added securely after you click "Send" as it passes through the mail server. Because of this, it won't appear in your compose window, but rest assured it is being cleanly attached to the final outgoing email.
How can I verify if an email is using embedded images?
You can check the raw code of a sent email to verify:
Open the sent email and view its original/raw source code (e.g., "Show original" in Gmail).
Search for
cid:orContent-ID.If your image sources look like
<img src="cid:image_name...">, the image is successfully embedded.
Which option should we choose?
Choose Hosted Images if: You want a lightweight email size, no attachment paperclip icons, real-time image updates, and signatures that persist cleanly through long reply threads (especially with iPhone and Mac users).
Choose Embedded Images if: Your primary audience uses desktop Outlook on Windows, and you want to guarantee your logo displays the very first time they open your email without them needing to click "Download pictures".