Hard as it might sound to admit, acs_mail_lite::complex_send is too slow to handle mass mailings and should probably not be used for notifications, though I will look into this matter.
What we just realized (the hard way) on a client site is that the moment they are sending mass notifications, complex_send goes down and takes with it the mailserver.
We might change the SMTP delivery functions in it to use ns_smtp functions just as acs_mail_lite::smtp is using, we are working in it then.
