TOBUFSIZE limits the total number of
recipients that can be delivered at once. It sets the size of the
buffer that will hold the list of recipients, where that default size
varies based on your operating system. If you need to increase that
limit you can experiment by cautiously increasing TOBUFSIZE.
To change the size of TOBUFSIZE, edit
sendmail/conf.h and rebuild
sendmail. There is no debugging switch that will
show the size of TOBUFSIZE.