Mimecast blocking images

Marketing & Content Hub Email Marketing Tool
1

Hi - mimecast seems to be blocking images in our emails, we are hosting them on our branded sub-domain. But they all come through as:

JWright76_0-1751466228337.png

We can potentially get IT to allowlist these but this will not solve the problem for others. The strange thing is the social images still get displayed, when I check these are still hosted on our sub-domain. The only difference I can really see is other images have url params attached e.g.
?upscale=true&width=600&upscale=true&name=Newsletter-connecting-us-2025-06-30.png
Any ideas what might be causing this and how can we resolve. There doesn’t seem to be a way to remove these params or even use an image hosted somewhere else.

2

Update: it doesn’t seem to be url paramaters causing the issue. I did a quick test using a custom module and it just seems to be images uploaded to the image library that are not displaying

JWright76_0-1751468447130.png
JWright76_0-1751468447130.png963×400 28.9 KB

JWright76_1-1751468479966.jpg
JWright76_1-1751468479966.jpg748×734 141 KB

3

More digging, I’ve run the image headers through GPT for a standard social icon (which displays) vs the same image but held in image libary. It seems there are some extra headers on library images which might possibly be causing Mimecast to block it.
I’ve also asked IT team if Mimecast gives any reason why they are getting blocked.

content-type image/png image/jpeg Normal difference
cache-control public, max-age=31536000 s-maxage=2592000, max-age=1209600, stale-while-revalidate=900 Social icon has simpler cache logic
timing-allow-origin :cross_mark: Not present :white_check_mark: Present → hubfs.cloudfront.hubspotusercontent-eu1.net :police_car_light: Possibly problematic
x-hs-alternate-content-type :cross_mark: Not present :white_check_mark: Present → text/plain :fire: Strong suspect
set-cookie :white_check_mark: Present (Cloudflare) :white_check_mark: Present Similar — not the differentiator
x-cache Hit from cloudfront Miss from cloudfront Working image likely pre-cached, lower latency
etag, last-modified :white_check_mark: Present :white_check_mark: Present No issue
content-length 3 KB 6 KB Not likely relevant
expires :white_check_mark: Present :cross_mark: Missing Might help email clients pre-load
server Cloudflare Cloudflare Same infra
4

Hi @JWright76 ,

Thank you for reaching out to the Community!

I wanted to invite our subject matter experts to this conversation. Hi @MikeCormack @karstenkoehler @Niharika - Do you have any advice for@JWright76 ? Thank you!

Best,

5

Hi @JWright76

Thank you for posting your question again.

The header x-hs-alternate-content-type: text/plain is almost certainly the reason Mimecast is blocking the images. While this header is part of how HubSpot’s infrastructure is configured to serve file assets efficiently, some aggressive security scanners can flag it. They see a resource identified as both an image and plain text and may block it as a security precaution against file-type spoofing.

Here are the official next steps I’d recommend:

  1. Get the Mimecast Block Reason: The most critical piece of information will come from your IT team’s request to Mimecast. Their admin logs will show the specific policy that is flagging the images.
  2. Open a HubSpot Support Ticket: I also strongly encourage you to open a support ticket with our technical team via your portal. Please include a link to this community post and the header analysis you’ve already done.

Thank you again for bringing this to our attention with such great detail. Please keep us updated on your progress.

Best,

Victor