印刷用テンプレートで引用を<blockquote>に変換して、CSSで左側に線を入れると引用を線で表示して印刷できます。たとえば以下のテンプレートをprint.templateとして保存するとそれらしくなります。
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>{@HtmlEscape(@Subject())}</title>
<style type="text/css">
body {{ margin-left: 3em; margin-right: 3em; }}
.header-title {{ font-weight: bold; font-size: 90%; }}
.header-value {{ font-size: 90%; }}
.body {{ font-family: monospace; }}
blockquote {{
border-width: 0px 0px 0px 2px;
border-color: blue;
border-style:solid;
padding-left:1em;
margin-left: 0.5em;
}}
</style>
</head>
<body>
<table border="0" cellspacing="0" cellpadding="0">
<tr>
<td colspan="2"><hr size="3" style="color: lightblue"></td>
</tr>
<tr style="padding-bottom: 5px;">
<td width="70" nowrap class="header-title">To: </td>
<td width="100%" class="header-value">{@HtmlEscape(@FormatAddress(To))}</td>
</tr>
{@If(Cc, @Concat('
<tr style="padding-bottom: 5px;">
<td width="70" nowrap class="header-title">Cc: </td>
<td width="100%" class="header-value">',
@HtmlEscape(@FormatAddress(Cc)),
'</td>
</tr>'), '')}
<tr style="padding-bottom: 5px;">
<td width="70" nowrap class="header-title">From: </td>
<td width="100%" class="header-value">{@HtmlEscape(@FormatAddress(From))}</td>
</tr>
<tr style="padding-bottom: 5px;">
<td width="70" nowrap class="header-title">Date: </td>
<td width="100%" class="header-value">{@FormatDate(@Date(Date), '%D %M1 %Y4 %h:%m:%s %z')}</td>
</tr>
<tr>
<td width="70" nowrap class="header-title">Subject: </td>
<td width="100%" class="header-value">{@HtmlEscape(@Subject())}</td>
</tr>
<tr>
<td colspan="2"><hr size="3" style="color: lightblue"></td>
</tr>
<tr>
<td colspan="2" class="body"><p>{-@Script(<<END
function getQuoteDepth(s) {
var depth = 0;
var n = 0;
for (n = 0; n < s.length; ++n) {
var c = s.charAt(n);
if (c == '>')
++depth;
else if (c == ' ' || c == '\t')
;
else
break;
}
return { "index": n, "depth": depth };
}
function escape(s) {
return s.replace(/&/g, "&").replace(/</g, "<");
}
var body = "";
var lines = arguments(0).split("\n");
var quote = 0;
for (var n = 0; n < lines.length; ++n) {
var line = lines[n];
var qd = getQuoteDepth(line);
var q = qd.depth;
if (q > quote) {
for (var m = 0; m < q - quote; ++m)
body += "<blockquote>";
}
else if (q < quote) {
for (var m = 0; m < quote - q; ++m)
body += "</blockquote>";
}
quote = q;
body += escape(line.substring(qd.index));
body += "<br>";
}
result.value = body;
END
, 'JScript', @Body('', :BODY-RFC822INLINE))-}</p></td>
</tr>
</table>
</body>
</html>