If you look at
/Applications/Safari.app/Contents/Resources/Safari.scriptSuite
you will see that Safari calls the AppleEventCode "mlct" which i assume stands for 'Mail Content'. Here is the Section in detail:
<key>EmailContents</key>
<dict>
<key>AppleEventClassCode</key>
<string>sfri</string>
<key>AppleEventCode</key>
<string>mlct</string>
<key>Arguments</key>
<dict>
<key>Target</key>
<dict>
<key>AppleEventCode</key>
<string>dcnm</string>
<key>Optional</key>
<string>YES</string>
<key>Type</key>
<string>BrowserTabViewItem</string>
</dict>
</dict>
<key>CommandClass</key>
<string>EmailContents</string>
</dict>
For testing purposes i have created a signature and changed the Font for viewing E-Mail to Calibri 14pt. After trying to send a Page from Safari i saved and exported the E-Mail-Draft as an .eml-File and opened it in a Text Editor. This is the output which i have indented for better reading purposes:
Content-Type: text/html;
charset=us-ascii
Subject: Apple
Mime-Version: 1.0 (Mac OS X Mail 6.0 \(1486\))
X-Apple-Base-Url: x-msg://50/
X-Universally-Unique-Identifier: ef77cd79-9fcf-447c-ada5-cec8a3969445
X-Apple-Mail-Remote-Attachments: YES
From: ***
X-Apple-Windows-Friendly: 1
X-Apple-Mail-Signature: 26ED4BEC-3AFB-4807-9E7F-EC7D356D0265
Content-Transfer-Encoding: 7bit
Message-Id: <[email protected]>
X-Uniform-Type-Identifier: com.apple.mail-draft
<html>
<head></head>
<body style="word-wrap: break-word; -webkit-nbsp-mode: space;
-webkit-line-break: after-white-space; ">
<span class="Apple-Mail-URLShareWrapperClass" contenteditable="false">
<span class="Apple-Mail-URLShareUserContentTopClass" style="font-family:
Helvetica !important; font-size: 12px !important; line-height: 14px
!important; color: black !important; text-align: left !important;
" applecontenteditable="true">Test<br>
<div id="AppleMailSignature">
<div>
<b>Test-Signature</b>
</div>
</div>
<br>
</span>
<span class="Apple-Mail-URLShareSharedContentClass"
style="position: relative !important; " applecontenteditable="true">
<a href="http://www.apple.com/de/">http://www.apple.com/de/</a>
<br>
</span>
<span class="Apple-Mail-URLShareUserContentBottomClass"
style="font-family: Helvetica !important; font-size: 12px !important;
line-height: 14px !important; color: black !important; text-align: left !important; "
applecontenteditable="true">
<br>
</span>
</span>
</body>
</html>
Several things are problematic with this output. It seems like the HTML Code generated by the Apple-Event is creating a CSS-Style with the attribute "!important" within a "span" Element. That means that regardless of your Settings in Apple Mail, 14pt Calibri in my case, it will override this setting and use Helvetica 12px.
The terminal command i mentioned in my first answer seems to be the only Way to adress the default font settings directly and to bypass the Apple Mail Settings.
For any other modfications a detailed analysis of the AppleEvent "mlct" would be necessary, but i don't know where to look for that. Suggestions anyone?