Emails on MAc have fonts changed

Emails from Mac are having fonts changed randomly. One user is complaining that everything seems okay on his Mac, however, he noticed that when people reply to his emails, his [art has different font and size to the one he used to send that email. We did some testing and it is definitely his machine. He is using Outlook 2011 for Mac and Mail. He has Mac OS X version 10.10.5. We are using Office 366 and he has Exchange account added to his email programs.
I was thinking about disabling Rich Text options on the email server and only leaving Plain Text and HTML.

How can I make sure that his emails look all the same?
Tom SkowyrskiAsked:
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William FulksSystems Analyst & WebmasterCommented:
What font is he using?

If it's something another computer does not have, then the software will try to substitute and results may vary.

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Qian BaoDigital Media Specialist and Web DesignerCommented:
I would suggest you to just use common font such as Arial or Times New Roman in outbound emails. Using plain text would be the best solution if you want the email to remain the same across different platforms.
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