Configuring Outlook to send HTML instead of Rich Text Format

If recipients are receiving your emails with missing images, broken signatures, or a strange attachment called winmail.dat, the cause is almost certainly that Outlook is sending your messages in Rich Text Format (RTF) rather than HTML.

Databias’ mail system, in common with the vast majority of modern email infrastructure, supports HTML format only. Rich Text Format is a proprietary Microsoft technology and is not an open email standard. When an RTF-formatted email leaves the Microsoft ecosystem, it either renders incorrectly or strips out elements like inline images and formatted signatures entirely.

This article explains why this happens, what TNEF is and why it must be disabled, and how to configure Outlook correctly on both Windows and Mac.

Understanding the three email formats

Before making changes, it helps to understand what each format actually is:

• Plain Text sends no formatting whatsoever; no bold, no colour, no images, no signatures. It is universally readable but visually bare.
• HTML is the open web standard used by virtually all modern email clients, marketing platforms, and mail servers. It supports images, formatted signatures, colour, fonts, and layout. This is the format our system is built around.
• Rich Text Format (RTF) is a Microsoft-proprietary format that predates the web. It supports rich formatting but only within the Microsoft ecosystem (Outlook to Outlook, on the same Exchange environment). The moment an RTF email is sent to anyone outside that ecosystem, Gmail, Apple Mail, a non-Microsoft server, the receiving system does not know how to render it correctly.

What Is TNEF and why is it a problem?

TNEF stands for Transport Neutral Encapsulation Format. It is the encoding mechanism Outlook uses when it sends Rich Text Format emails.

When Outlook sends an RTF message, it encapsulates all the formatting, images, and metadata into a binary blob and attaches it to the email as a hidden file. That file is named winmail.dat. The actual email body is stripped back to plain text, and all the “rich” content, including inline images and your HTML signature, is packed into the winmail.dat attachment instead.

Recipients on non-Outlook clients receive an email that looks like this:

1. Body text only, with no formatting
2. Inline images missing or not displayed
3. An inexplicable attachment called winmail.dat that they cannot open
4. An HTML signature that is either missing or appears as raw code

From their perspective, your email looks broken. From your perspective, it looked perfectly fine before you sent it.

TNEF / winmail.dat is the most common cause of image stripping in professional email environments. Disabling TNEF is not optional, it must be turned off for outgoing mail to render correctly outside Microsoft environments.

Configuring Outlook on Windows

Setting HTML as the Default Format for All New Messages

1. Open Outlook and click File → Options.
2. In the left panel, select Mail.
3. Under the Compose messages section, find the dropdown labelled Compose messages in this format.
4. Change the selection to HTML.
5. Click OK.

This ensures all new messages you compose default to HTML.

Preventing Outlook from overriding HTML when replying

Outlook can override your default format when replying, it will match the format of the original message. If someone sends you a plain text email, your reply will also be plain text. To fix this:

1. Go to File → Options → Mail.
2. Under Reply messages in this format (in some versions this appears under Compose messages), ensure HTML is selected.
3. Click OK.

Disabling TNEF (Eliminating winmail.dat)

This is a separate setting and must be configured explicitly.

Method 1 – Via Outlook Options (Outlook 2016 and later):

1. Go to File → Options → Mail.
2. Scroll down to the Message format section.
3. Find the setting When sending messages in Rich Text format to Internet recipients and set it to Convert to HTML format.
4. Click OK.

This instructs Outlook to convert any RTF message to HTML before it leaves your outbox, preventing TNEF encoding and eliminating winmail.dat.

Method 2 – Per Contact (if a specific recipient keeps receiving winmail.dat):

If a particular contact consistently receives winmail.dat despite your global settings, it may be because their address is stored in your Contacts with a forced RTF setting. To fix this:

1. Open the contact in your Contacts / People folder.
2. Double-click to open the full contact record.
3. Go to File → Properties (or right-click the email address and choose Properties).
4. Under Internet format, change the setting to Send Plain Text only or Send using Outlook Rich Text format — and then change it to Let Outlook decide or HTML.
5. Save the contact.

Configuring Outlook on Mac

Outlook for Mac has a slightly different interface but the same underlying requirements.

Setting HTML as the default format

1. Open Outlook and go to Outlook → Preferences (or Outlook → Settings on newer versions).
2. Click Email (or Composing).
3. Under Compose messages, find the Message format or Default format dropdown.
4. Select HTML.
5. Close Preferences.

Disabling Rich Text / TNEF on Mac

Outlook for Mac does not use TNEF in the same way as the Windows version, as it is not typically connected to an on-premises Exchange server in the same configurations. However, you should still verify:

• In Outlook → Preferences → Email/Composing, confirm the default format is HTML and not Rich Text.
• When composing a message, check the Format menu in the menu bar — it should show formatting options consistent with HTML, not RTF.

If you are connected to a Microsoft Exchange or Microsoft 365 environment, contact your IT administrator to confirm that the Exchange server is not enforcing RTF conversion on outbound messages, this is a server-side setting that overrides client configuration.

Checking the format of an individual message

Before sending any important message, you can verify the format directly in the compose window:

• On Windows: In the compose window ribbon, click the Format Text tab. The active format will be highlighted – confirm HTML is selected, not Rich Text or Plain Text.
• On Mac: In the compose window, go to the Format menu in the top menu bar and look for the message format options.

If you ever see Rich Text highlighted or selected, click HTML to switch before sending.

A Note on why our system does not support Rich Text

Rich Text Format / TNEF was designed in the 1990s for internal Microsoft Exchange environments where every sender and recipient used Outlook on the same server. In that closed ecosystem, it works. Outside it, it does not.

Databias’ mail system, like Gmail, Apple Mail, and virtually every non-Microsoft platform — speaks HTML. HTML is the open, universal standard for formatted email. It is what powers email signatures, inline images, branded templates, and every marketing or transactional email you have ever received.

Sending in RTF to our system is the equivalent of sending a document in a format the recipient’s software cannot read. The server does its best to interpret it, but the result is inevitably degraded — which is why images disappear, signatures break, and mysterious attachments appear.

Configuring Outlook to send HTML is a one-time change that eliminates these issues permanently.