When you initiate emails from your iPhone (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.
Set the default signature on the iPhone to bold or italic.
You can also add a space which you mark as bold.
With these changes, all emails sent from your iPhone that include this signature will be sent in HTML.
- On the iPhone tap Settings > Mail, Contacts, Calendars and scroll down to the Signature section.
Tap Signature and then tap and hold on your current signature and tap Select All.
Tap the > arrow and then select the B/U button.
Make the text bold or italic and then tap Mail (top left) to save the settings.