I'm not sure you can control it. See in Outlook, we are able to enjoy richly
formated emails that may include decorative backgrounds and fancy, colorful
lettering . And as long as the recipient is also using Outlook they may also
enjoy your richly formated emails too.
However, if your email if received by someone who is not using Outlook, how
has restricted incoming emails in Outlook or their email system does not
support or restricts, or strips messages down to plain text, then the crazy
formatting of from replied emails begins.
The challange is you can't control other peoples email systems or the
formats that it supports.
However, for some control, try changing some of your email options in Outlook.
For incoming emails, try Tools: Options: Preferences tab: Email Options:
Mail Handling section: check Remove extra line breaks in plain text messages.
For outgoing emails, try Tools: Options: Mail Format tab.