When you initiate emails from your iPhone using Mail app (iOS) it results in plain text format on the receiving end.
By default, an iPhone will always send new messages in Plain Text unless any formatting has been applied to the text.

Many apps don’t support rich-text formatting. Pasting text into one of those apps strips all styles and fonts. You can then copy that same section of text, and it’s unstyled. 3: Strip Text Styling & Formatting with TextEdit. TextEdit the simple text editing app that is included in all versions of Mac OS X, and you can use it’s built-in rich text conversion abilities to strip formatting very quickly. Here’s all you need to do.

If formatting is applied to the text, such as making the text bold or italic, then it will send the message in HTML.

Apps For Formatting Rich Text Newsletters For Mac Mail Merge

How to change the email to HTML at the recipient's end

You can change to HTML after the email is sent from iOS devices.
The Plain Text signature is applied before the email is converted to HTML.
First, Open 'Preferences' menu, select 'Integration' tab.
Click 'Start Integrate' to connect to your Office 365.
Then select 'Convert Plain Text emails to HTML for iOS devices' and sent an email to an external contact:

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