If you're talking about within the code...  It knows the mailbox by the IMAP socket connection, which is in the $IMAP object, specifically: $IMAP->get_mailbox_name() which will changed based on what mailbox is selected.
<br><br><br><div><span class="gmail_quote">On 3/29/06, <b class="gmail_sendername">Paul Cottrell</b> &lt;<a href="mailto:pcottrell@borran.com" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">pcottrell@borran.com
</a>&gt; wrote:</span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"><div style="direction: ltr;"><div>when someone goes to view a message from either thier inbox or sent items, it goes through /steps/mail/show.inc, my question, is there a way to&nbsp;know which mailbox that message resides in?
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<div>Thanks</div></div><div style="direction: ltr;"><span>
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<div>Paul</div>

</span></div></blockquote></div><br><br clear="all"><br>-- <br>&quot;A conclusion is simply the place where someone got tired of thinking.&quot;