Other: Write it before the body: it's a headline.
Your message may be important, but the Subject will determine when (and sometimes if) I read it. I always work with the preview pane off, so the Sender, Subject and Date may be the only part of your message I ever see. Of those, the Subject is the one you control for any given message, so make it count.
Once I've opened the message, the Subject doesn't really matter, other than to track and group the conversation, depending on the mail reader.
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Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
<code> <a> <b> <big>
<blockquote> <br /> <dd>
<dl> <dt> <em> <font>
<h1> <h2> <h3> <h4>
<h5> <h6> <hr /> <i>
<li> <nbsp> <ol> <p>
<small> <strike> <strong>
<sub> <sup> <table>
<td> <th> <tr> <tt>
Snippets of code should be wrapped in
<code> tags not
<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
Want more info? How to link or
or How to display code and escape characters
are good places to start.