Just a quick question here. I work for a focus group testing company. We keep email addresses from thousands of participants nationwide and we routinely email them for upcoming groups. Unfortunately the only email program we have for right now is Outlook Express at each individual branch. Our IT Director has stated that we cannot send more than 1,000 emails out at a time, a number I found odd. I thought that big email providers like major ISP's, Yahoo, MSN, AOL etc.... blacklisted any email that was sent to over 350 recipients. I really thought I read this somewhere, but I can't seem to find the article again. Any help would be appreciated.
Many ISPs work differently and will rate your email as "spam" (or "not spam") on many factors, including things such as how quickly you are sending mail to their servers, how often/many people have marked your email as spam, and of course various tell-tale signs that might mean your message is spam. Using software dedicated for bulk mailing will definitely help. You may wish to take a look at the software in my signature.
Unfortunately I am not in charge of software procurement or a bulk mailer program would be something I would get, I will suggest it to teh IT director. What we are really looking for at the moment is just an average number of recipients that will be considered 'tell-tale' signs of a spammer so we can avoid being blocked.
I would keep that number very low. (~100) Many ISPs will start auto blocking. Even if you think email went through successfully they may not have. Bulk mailer is the only way to go. Heck you can get Send Blaster (my personal favorite) for like $100 or less.