OK, don't have time for the exact code, but this should get you looking in the right direction:
1) Pull all your fields from the database
2) Create a form tag
3) Cycle through all your records, output a checkbox and the email. Give each checkbox a unique name.
4) Create the form end tag at the end of your loop
5) When you submit form:
6) Cycle through form checkbox variables
7) If checkbox is on, add email plus ";" to a string.
8) Mailto string
When creating your checkboxes, use the count variable in the name.
for ($i = 0; $i < $numRows; $i++)
{
print "<input type=\"checkbox\" name=\"email{$i}" value=\"ON\">" . mysql_result($result, $i, 'email');
}
Cycling through form tags:
if ($_POST[$email{$i}] == "ON")
{
$string .= $value . ";";
}
Hope this makes sense. Not a direct answer, but just a nudge in the right direction.