Do you have access to the POP3/IMAP server the recipient will be using to check their mail? You can parse server logs to see when they check their mail and where they were coming from - although this will generally only tell you that they logged in, without telling you which messages they downloaded. A Webmail system will also have logs, and may indicate individual messages somehow.

You can send e-mail in HTML format with an <img> tag which will load a small graphic on a web server you control (you can copy http://phroggy.com/graphics/null.gif if you want), and make a CGI script to serve the graphic and send you notification, for example:

#!/usr/bin/perl
# send notification here
my $gif=`cat null.gif`;
print "Content-type: image/gif\n\n$gif";
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However, be aware that because this technique is frequently abused by spammers, all decent e-mail clients give the option to disable loading images this way.

Finally, consider sending a message with a link to a web page that the recipient must click on in order to read your message. The link they click can take them to a CGI which will send you notification - but you won't get automatic notification that they've received the e-mail itself, just that they've clicked the link in the e-mail (which they aren't guaranteed to do).

for $a(-2,12){for $b(0..7){$c=0;$_?hex substr(ef7fa1866706caeff0228940
+2844,
2*$_+$a,2)&2**(7-$b):0 and $c+=2**(7-$_)for(0..7);print chr $c;}}
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