Are my lists visible to recipients?

Absolutely not, you can be fully confident that your lists will never be sent to your recipients. By default MaxBulk Mailer sends your messages individually, in other words, it sends them to each recipient one by one. This is how the 'Singly' mode works, it is like if you were sending yourself the messages manually one by one with Outlook, Mail or Entourage. The recipients will always receive fully customized e-mails with their data properly placed in the To header exactly if they were addressed only to them. Actually it is impossible to distinguish a message sent with MaxBulk Mailer from one sent with Outlook, Mail, Entourage or any other e-mail software. In addition, because e-mails are sent one by one, you can use tags to insert custom recipient's info inside the subject or the body of the messages.

Now, if you uncheck the 'Singly' box, the whole list is sent to your server but remains hidden to the recipients thanks to a custom To header set to 'undisclosed recipients'. Actually thanks to that header the list is not sent to your recipients at all so again you can be fully confident that your lists will never be made public. This is Bcc mode (aka bulk mode), a delivery mode highly recommended when sending big attachments however it doesn't support the use of tags.