This code snippit from horde.lib (www.horde.org):                                              
                                                                                
 // Finally, send the mail.                                                     
        if (@is_executable($default->path_to_sendmail)) {                       
                $mail = popen("$default->path_to_sendmail -i -f$from --  $recipients", 'w');                                                             
                $result  = fputs($mail, $bhdrs . $hdrs);                        
                echo "1: " . $result;                                           
                $result += fputs($mail, "\n");  // trailing \n to end the       
headers section                                                                 
                echo "2: " . $result;                                           
                $result += fputs($mail, $body);                                 
                echo "3: " . $result;                                           
                if (pclose($mail) != 0)                                         
                        $result = 0; 
	  echo "4: " . $result;                                         
        } else {                                                                
                if (empty($envelope['To'])) $envelope['To'] = '';               
                if (empty($envelope['Subject'])) $envelope['Subject'] = '';     
                if (mail($envelope['To'], $envelope['Subject'], $body, $hdrs .  
 $mhdrs))                                                                        
                        $result = 1;                                            
                else                                                            
                        $result = 0;                                                                                       
        }                                                                       
                                                                                
Yields the following output:                                                    
1: 345                                                                          
2: 346                                                                          
3: 354                                                                          
4: 0                                                                            
                                                                                
The return value of pclose() is not the exit status of the program, but a value  as returned by waitpid() of wait4().  I've run what the popen command-line calls by hand using truss, and sendmail is returning exit(0).  So, I dug a bit further... pclose($mail) is returning -1.  Any ideas on how I can track down why?  This function is not reported to be misbehaving on other platforms.