<font face="Times New Roman, Times, serif">Times</font><br>
<font face="Helvetica, Arial, sans-serif"><br>Arial</font><br><br>
<font face="Courier New, Courier,monospace">Courier</font><br>

It would be interesting to see the html source of the message (as received) to see if it actually specifies a font. I have had similar experiences myself and found that the font typeface wasn't explicitly given. If the font really matters then I'd suggest an add-on that lets you set explicitly the size and the face, or use Stationery to apply styles to your messages. https://codefisher.org/projects/extra_format_buttons/ https://addons.mozilla.org/en-US/thunderbird/addon/stationery/ Using the toolbar add-on, my sent message has this code: <code> &lt;font face="Times New Roman, Times, serif"&gt;Times&lt;/font&gt;&lt;br&gt; &lt;font face="Helvetica, Arial, sans-serif"&gt;&lt;br&gt;Arial&lt;/font&gt;&lt;br&gt;&lt;br&gt; &lt;font face="Courier New, Courier,monospace"&gt;Courier&lt;/font&gt;&lt;br&gt; </code> and you can see it explicitly sets the typeface to be used.