We're not CB Juices developer. Your question is probably better asked in CB Juice boards below.

www.joomlapolis.com/forum/226-cb-juice

My best guess is it handles as many users as your server can handle before timing out. Obviously importing 50,000 users at once is not going to work unless you've a very powerful dedicated server.

I recommend importing in chunks of 300 users per import at most. Yes this applies to localhost servers. Most by default will timeout after 30 seconds and at most allow up to 100MB of memory. You'd need to adjust your localhost to have a ridiculously high timeout time and allow for much more memory (via PHP.ini).

None of which is CB Juices fault of course. You're just performing way too much all at once.

Why am I being emailed every failed email after I have selected the do not email the addressee function. That would also mean that the valid emails also got sent. Is there a way to turn this off. I do not want my email box filled with 50,000 invailid.com return emails.

I don't know. Again, we're not CB Juices developer and you may want to ask your question on CB Juices boards below.

www.joomlapolis.com/forum/226-cb-juice

Thank you. I would be willing to pay for a module that was not giving me these problems. Any hope for this at CB. Or is there a more professional application that works with CB?

CB Juice works fine and as I've stated already the problem is your unrealistic imports. A server can not handle that many imports no matter who develops it. You're literally running thousands of users through the registration process at once, which will kill your server (localhost or not, they all have limitations).

We are planning an official import/export feature built directly into CB, but it won't be until late CB 2.0 at best.