It could be how you're sending your emails out of Outlook for Mac.
MS made a weird decision on how emails are sent from Outlook for Mac. And that's, even if you have HTML turned on for composing, if you don't make something in the email other than straight up text, it will be sent as plain text! Just changing the font doesn't change that. At least, not if it's all the same font.
So you have to mark something, anything, that would force the email to be sent as HTML. Such as marking a piece of text bold, using more than one font, or adding a small graphic. Then your recipients will see the text as Arial. Otherwise, they receive the email as plain text and it displays on their end as whatever they have set as the default font for plain text.