Ok, on further research it doesn't seem to be as simple as I thought. It's hard for me to fully check now, as I've installed Universal Mailer, but it seems as if this might only apply to mail with a pre-formatted signature.
I've pasted the relevant part of the html from an affected email below. For obvious reasons I've replaced most of the text with asterisks, but as you'll see, some of the text (in italics) is in Verdana (in this case the first two lines), and some (in bold) isn't specified, and appears in whatever default font I choose. (I've added line breaks for clarity.)
<html>
<head>
<meta http-equiv=3D"Content-Type" content=3D"text/html =
charset=3Dus-ascii">
<base href=3D"x-msg://185/">
</head>
<body =
style=3D"word-wrap: break-word; -webkit-nbsp-mode: space; =
-webkit-line-break: after-white-space; ">
<div =
apple-content-edited=3D"true">
<span>
<img height=3D"62" width=3D"297" =
id=3D"dc979ff9-d9ed-4b0f-9ea6-4f6cadf12218" apple-width=3D"yes" =
apple-height=3D"yes" src=3D"cid:3451943488_157568">
</span>
<span =
style=3D"color: rgb(0, 0, 0); font-family: Verdana; font-size: 12px; =
font-style: normal; font-variant: normal; font-weight: normal; =
letter-spacing: normal; line-height: normal; orphans: 2; text-align: =
-webkit-auto; text-indent: 0px; text-transform: none; white-space: =
normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; =
-webkit-text-stroke-width: 0px; ">
<br>
<br>
<br>Hi M*****,
</span>
</div>
<div =
apple-content-edited=3D"true">
<span style=3D"color: rgb(0, 0, 0); =
font-family: Verdana; font-size: 12px; font-style: normal; font-variant: =
normal; font-weight: normal; letter-spacing: normal; line-height: =
normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; =
text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; =
-webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; =
">
<br>
</span>
</div>
<div apple-content-edited=3D"true">
<span =
style=3D"color: rgb(0, 0, 0); font-family: Verdana; font-size: 12px; =
font-style: normal; font-variant: normal; font-weight: normal; =
letter-spacing: normal; line-height: normal; orphans: 2; text-align: =
-webkit-auto; text-indent: 0px; text-transform: none; white-space: =
normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; =
-webkit-text-stroke-width: 0px; ">Thank you for your =
reply.
</span>
</div>
<div apple-content-edited=3D"true">
<span =
style=3D"color: rgb(0, 0, 0); font-family: Verdana; font-size: 12px; =
font-style: normal; font-variant: normal; font-weight: normal; =
letter-spacing: normal; line-height: normal; orphans: 2; text-align: =
-webkit-auto; text-indent: 0px; text-transform: none; white-space: =
normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; =
-webkit-text-stroke-width: 0px; ">
<br>
</span>
</div>
<div =
apple-content-edited=3D"true">
<span style=3D"orphans: 2; text-align: =
-webkit-auto; text-indent: 0px; widows: 2; ">Just to clarify: *********************************?
<br>
<br>Kind =
regards,
<br>
<br>*****
<br>________________
</span>
<font size=3D"2" =
style=3D"color: rgb(0, 0, 0); font-family: Verdana; font-style: normal; =
font-variant: normal; font-weight: normal; letter-spacing: normal; =
line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: =
0px; text-transform: none; white-space: normal; widows: 2; word-spacing: =
0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; =
">
<span style=3D"font-size: 10px; ">
<br>
</span>
</font>
<span =
style=3D"color: rgb(0, 0, 0); font-family: Verdana; font-style: normal; =
font-variant: normal; font-weight: normal; letter-spacing: normal; =
line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: =
0px; text-transform: none; white-space: normal; widows: 2; word-spacing: =
0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; =
font-size: 10px; ">
<font color=3D"#808080">
<h5>***** *****
<br>**********
<br>
</h5>
</font>
</span>
<div apple-content-edited=3D"true" =
style=3D"color: rgb(0, 0, 0); font-family: Verdana; font-size: medium; =
font-style: normal; font-variant: normal; font-weight: normal; =
letter-spacing: normal; line-height: normal; orphans: 2; text-align: =
-webkit-auto; text-indent: 0px; text-transform: none; white-space: =
normal; widows: 2; word-spacing: 0px; -webkit-text-size-adjust: auto; =
-webkit-text-stroke-width: 0px; ">
<font>
<span style=3D"color: rgb(128, =
128, 128); font-size: small; ">
<a =
**************
</a>
</span>
</f=
ont>
</div>
<div apple-content-edited=3D"true" style=3D"color: rgb(0, 0, =
0); font-family: Verdana; font-size: medium; font-style: normal; =
font-variant: normal; font-weight: normal; letter-spacing: normal; =
line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: =
0px; text-transform: none; white-space: normal; widows: 2; word-spacing: =
0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; =
">
<font color=3D"#808080" size=3D"2">Tel: **************
<br>Mob: *************
<br>****************
</font>
</div>
</div>