HTML emails are bad for a few reasons.
Firstly they are heavily bloated. Why include a few kb of code to say hi when you can do it in a couple of bytes.
Secondly, HTML can be used to call code from an external source, for example an image held on a server. This can be used to verify if an email has been read, and exactly which IP address it was sent to, a significant security risk, and the reason why many people have their email clients set to read in plain text only, so the external code is not read.
Thirdly, Word is a seriously bad HTML editor, so bloats the emails even more.
And lastly, some email clients cannot read HTML emails at all, so your missive will not be read.
Turn off Word as your email editor, it will save memory and CPU cycles on your machine, and reduce the size of any emails you send. If you still want to use alternative fonts in emails set Outlook to send emails in rtf format.