email recipients using mail app on Mac not able to load my logo on signature.

Question

When I send emails, the logo in my signature does not display for the recipient and instead appears as a winmail.dat attachment. I’ve tested this by emailing from outlook to a Gmail account - using the Mail app to receive the email and encounter the same issue. I’ve already removed all privacy restrictions in the Mail app and confirmed that remote content loads correctly from other senders—however, this problem persists specifically with emails sent from my Microsoft 365 account.

My settings are set for HTML for composing messages.

Thanks

Answer 1

Hi Jason F

Thank you for reaching out via the Microsoft Q&A forum.

According to your description, I understand that this is a classic conflict known as the TNEF (Transport Neutral Encapsulation Format) issue.

Even though your settings are set to HTML, the Microsoft Exchange server is encapsulating your email (and the embedded logo) into a proprietary format that Microsoft Outlook understands perfectly, but Apple Mail does not. When Apple Mail receives this bundle and can't unpack it, it presents it as a winmail.dat file.

For detailed information, you can see: TNEF conversion options | Microsoft Learn

Here are some recommendations from my side, and I hope they will help.

Change the Setting in the Exchange Admin Center

Since this is a business account, the issue is likely occurring at the server level before the email even leaves your organization. If you are IT Admin, you need to tell the server never to send TNEF encoded messages to external recipients (if you do not have admin access, please ask your organization's IT department for help).

• Log in to the Exchange Admin Center with admin account > In the left-hand menu, go to Mail flow > Remote domains.
• Click on Default > click Edit text and character set.

Look for the setting Use rich-text format (or Use TNEF) > Change this setting to Never > Save.

(Note: It may take up to an hour for this change to propagate)

Copy/paste an image directly:

I also tested copying and pasting directly into New Outlook on my Mac, and it worked as expected. Here are the steps you can try:

• Upload your logo to a public web server (your company website’s media library, or a cloud drive with a direct public link).
• Open the image in a web browser, right-click it, and select Copy Image
• Open Outlook > Settings > Signatures > create a new signature and paste an image directly (Command + V)

Additionally, if it is an emergency situation and you need to fix it immediately, you can consider reverting to Legacy Outlook (Diagnostic) temporarily.

I hope this information is helpful.

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Answer 2

That worked! Thanks so much for the answer and clarity.