Outlook HTML email signature - recipient images are blank

Kieran Stafford 0 Reputation points
2026-09-16T14:41:59.13+00:00

User's imageUser's imageUser's image

Hi,

I’m having an issue with an HTML email signature in Classic Outlook (please see images attached).

The signature contains several images. When I send an email, the images are sometimes received correctly, but on fewer other occasions the recipient sees blank spaces or red crossed image boxes instead.

I’d like to understand whether this could be caused by the images being blocked or stripped by Outlook, Microsoft 365, or the recipient’s mail server.

I’ve also noticed that when I manually insert an image into an Outlook signature, Outlook appears to embed it using a CID/content ID rather than simply linking to the image file.

Would using CID-embedded images in the signature bypass the issue if the recipient’s mail server is blocking or removing externally/local referenced images? If so, is there a recommended way to set up an Outlook signature so that all images are automatically embedded when the email is sent, without having to manually reinsert them each time?

Lastly, is it better to use the signature editor inside outlook rather than input the HTML into: C:\Users<user name>\AppData\Roaming\Microsoft\Signatures

Outlook | Windows | Classic Outlook for Windows | For business
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  1. Helen Luu 2,945 Reputation points Independent Advisor
    2026-09-16T16:00:36.0866667+00:00

    Hello @Kieran Stafford

    Could you please let me know which version of Outlook you are using? To check this, please open Outlook and navigate to File > Office Account (or File > Account) and share the complete version and build number shown under About Outlook.

    Based on your description, this behavior could be related to how the images are referenced in the HTML signature. In some cases, Microsoft 365 security policies, email security gateways, or the recipient's mail server may block or prevent the download of externally linked images, which can result in blank image placeholders or red X icons appearing in the message.

    If the signature appears blank or some images are missing, could you please confirm whether the same behavior occurs across all Outlook platforms, such as Outlook on the web, Outlook classic and new Outlook?

    Additionally, using CID-embedded images (Content-ID embedded images) can often be more reliable than externally linked images because the images are included within the email itself rather than being downloaded from an external location. However, while this approach may reduce issues caused by image blocking, it does not guarantee that images will display correctly in every email client or environment.

    You should manually remove and reinsert each image in the signature one by one, save the changes, and then have a test email. Please let me know whether the issue still occurs after re-adding the images.

    I look forward to your update.

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  2. AI answer

    2026-09-16T14:42:26.2166667+00:00

    Yes—what is shown is consistent with Outlook image rendering behavior, and the failure can happen for different reasons depending on how the images are referenced.

    Key points:

    1. Externally referenced images can fail to display Classic Outlook can block or fail to render linked images. Microsoft documents cases where invalid IMG SRC values cause Outlook to drop the image during HTML import, and file: image sources are treated as not secure.
    2. CID-based images are also not guaranteed in every scenario Microsoft has documented a classic Outlook issue where images referenced with cid: in message source were not rendered after certain updates. That means CID embedding does not guarantee that images will always display.
    3. Rich Text and HTML formatting matter In classic Outlook, if a message is sent as Rich Text, attachments can appear in the message body. For image-heavy signatures, HTML format is the safer format.
    4. There have been Outlook-side bugs affecting signature images Microsoft has documented multiple Outlook issues where images in signatures, newsletters, or embedded content showed as blank or red X, including cases fixed by later builds.

    What this means for the scenario:

    • If the signature uses external or local file references, the recipient may see blanks or red X if Outlook treats the source as invalid or insecure.
    • If the signature uses CID/content-ID images, that can help avoid dependence on external image download, but it still does not fully bypass Outlook rendering issues because classic Outlook has had known CID-related display bugs.
    • The problem is therefore not limited to the recipient’s mail server. It can also be caused by Outlook client behavior.

    Recommended setup based on the documented behavior:

    1. Use HTML message format for the signature emails.
    2. Do not use local or UNC/file-based image paths in the signature HTML.
    3. If manually inserting pictures in Outlook, that aligns with Outlook’s normal signature workflow and can create embedded image references, but there is no documented method here that guarantees Outlook will automatically convert every HTML signature image into a reliable embedded image at send time.
    4. If the issue is specifically the classic Outlook error “The linked image cannot be displayed”, Microsoft documents a registry-based fix by removing BlockHTTPimages.

    Important: modifying the registry is risky. Back up the registry first. The documented steps are:

    • Exit Outlook.
    • Open Registry Editor (regedit.exe).
    • Check one of these locations for BlockHTTPimages:
      • HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Common
      • HKEY_CURRENT_USER\Software\policies\Microsoft\Office\x.0\Common
    • Delete the BlockHTTPimages value.
    • Restart Outlook.

    If the value is under the Policies hive, it may be enforced by Group Policy and will return unless the policy is changed.

    On whether to use the Outlook signature editor or edit the signature files directly:

    • The available documentation here supports that Outlook’s built-in signature workflow is the normal path for inserting pictures into signatures.
    • There is no supported guidance here stating that editing the signature HTML files directly is better or more reliable for automatic image embedding.
    • Based on the documented Outlook behavior, using the signature editor inside Outlook is the safer approach.

    References:

    AI-generated content may be incorrect. Read our transparency notes for more information.

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