Well, one thing you should know is that some e-mail providers limit the amount of bulk mail you can send out in a certain time period.
And there is no cathc-all trick to avoid hte spam filter. There may some tricks, but you should stick with a known e-mail provider, interlligable subjects lines that don't use spam words like "won", "viagra", "Uk lottery", "please contact immediately", and you can record data from people to seee which providers filter your e-mai las spam, then see if those providers offering safe word filtering, where certain e-mail providers or words can be makred as "Safe" and sent to the Inbox, then mention that on your site.
What is hte business, by the by?