What's the best way to send emails and have a high success of the email reaching the users Inbox (not spam). I have a list of 40,000 email addresses from one of my sites (they ticked the box saying I can email them), the problem is that this isn't a forum, it's a custom coded site and I simply have a 'list' of them. At the moment I'm using something called "Mass eMailer" which I purchased off the net, but it can be slow and tends to be picked up as spam a lot of the time. Any ideas/applications/scripts I can use?
The software you are sending from is highly unlikely to have any effect on where the email is delivered. Think of it this way: if you have an awesome telephone that lights up and vibrates, it doesn't affect whether or not your friend will answer his phone. Get your list to double opt-in. So you send them an email when they sign up, and they have to click a link in the email to confirm that they want to be on your list. This usually means that they'll whitelist your address to make sure the emails always get through, or that they'll look out for them so they don't get flagged as spam.
Now days all the big email isps like yahoo.com and hotmail.com use server IP reputations to help deliver good emails and ban spam mails. Your best off going with an established ESP that is hosted.
Is not the software guilty for the poor deliverability of your emails. Here are some tips for improving emails deliverability: http://ezinearticles.com/?Email-Newsletters-Deliverability-Tips&id=2182150