Sorry, but we don't add new features to IMP 3.x. A patch including 
configuration against CVS HEAD would be considered though.

i had a case were i wanted to restrict the maximum number of 
recipients so as to stop spammers abusing my webmail accounts. I 
wanted to restrict the total number of TO, CC and BCC recipients as 
spammers were adding thousands to teh BCC field and hitting send.



my solution may not be refined, but it works.



in compose.php in the SEND_MESSAGE case right after $recips is defined 
and addresses assigned I added the following code



         if (substr_count($recips,",") + substr_count($recips,";") + 
substr_count(ereg_replace("  "," ",$recips)," ") > 21) {

          $get_sig = false;

          Horde::raiseMessage(_("For security reasons you can not send 
an email to more than 22 persons at once. If you need to sent to more 
than 22 persons, please create a second email."), HORDE_ERROR);

          break;

      }



this may be useful, and the variable 22 could be added to conf.



Thanks.