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.
If formatting is applied to the text, such as making the text bold or italic, then it will send the message in HTML.
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:
How to change Mail (iOS) to send in HTML format
When you add a space and format as bold, then the iOS device will send email in HTML format instead of its default plain text format. Therefore HTML signature is applied.
On the iPhone tap Settings > Mail > Signature and scroll down to the Signature section.
Tap Signature and then insert a space.
Select the space and then select the B/U button.
Make the text bold or italic and then tap Mail (top left) to save the settings.