I want to recommend fmSpark to a new client I have but they send out thousands of emails a month in email campaigns and use Constant Contact. Is there a good mass email SMTP service(s) in the Internet that would allow that many emails and not have a problem with blacklisting?
Anyone have any recommendations for that type of email hosting?

Thanks.

I'm not sure what Constant Contact is...but I'm guessing it is your clients own list of people.

Most the mass SMTP services out there offer to maintain your contacts for you! This is not an offer, they have to maintain that list (as well as the opt out policies) in order to be in compliance with anti-spam laws.

The one place that I have found that will allow their customers to use their own contact list (though they still have to provide verification that the emails are valid and "opted in" ) is authSMTP. Their website is at
http://www.authsmtp.com