If there aren't any new details i would try to offer some details in the dark:
I believe that bulk emailing should be done only on a known list because otherwise it could hurt your brand or product.
The first thing is to identify that your users are actually real people and should check your database with an email validation system in order to get rid of spam traps, invalid emails and all sorts of incorrect emails.
Secondly, try to offer good content or good/real offers in order to attract users that have a real intent about what you have to offer.
If available (if not.... try) try to segment your list in order to increase your return.
The last thing try not to send just plain image emails because most mail systems usually block images at first.