How to change message format from plain text to HTML in Outlook?

If you wish to set HTML as the default format for all the outgoing emails from your Outlook, please follow the below steps in your Outlook:

Open your Outlook and click the File button top left corner.
Choose Options > Mail.
Under Compose messages, in the Compose messages in this format list, choose HTML and hit OK to save the changes.

If you want to change the email format to HTML to a particular email only, please follow the below steps while sending that email from your Outlook:

Click on New message in your Outlook to draft the email.
Click on Format Text in the Menu bar.
Under the Format Text, you will get an option to toggle between HTML text, plain text, and Rich text.
You can choose the HTML option and then send your email.

Below is the definition of each format and how it affects your email:

HTML: It's a default message format set in Outlook (unless you change the settings). It's strongly suggested to use HTML format for your emails as it allows you to add various fonts, colors, and bulleted/numbered lists & pictures in the emails. It's by far the most suitable format for emails.

Plain text: Although this format is supported by every email program, it may not support formatted text like bold or italic. This format also doesn't allow you to embed images in the emails. You can include pictures as attachments though.

Rich Text Format: It's Microsoft email format which is supported by all Outlook versions and Microsoft Exchange Clients (versions 4.0 and 5.0). You can use Rich Text Format in your emails when you're sending emails within an organization that uses Microsoft Exchange. It's an optional format and users are always recommended to use HTML format. If you send emails outside the organization, the email format is converted to HTML by Outlook automatically.