sending mail with PHP from web

Hi there,... i had make a web interface to send announcement mail to my customers,..rite now i had about 500 once I send mail to my customers my server hang ( i guess to much open file)...i complie by kernel again to get a big descriptor, but still worried about the hang problem....i put a delay in my program so it send a mail and delay in x time, but i get a web error and my mail stop sending in my customer number 300 sorry for the broken english can any one help me with this kind of problem... i get my customer from mysql table, using freebsd4.1 and php4 Yamin Prabudy