That message means the plugin did successfully send the email subscriber to MailChimp and MailChimp confirmed it was received. From there it is “out of our hands”.
Note that the double opt-in setting in your MailChimp account is overruled by the setting on the form within our plugin, so please check MailChimp for WP > Form > Settings to see if double opt-in is turned on or not. If it’s turned on there, contacts won’t show up until they click the confirmation link in their email. The opt-in email is send by MailChimp, not by the plugin. It could end up in your spam.
When double opt-in is turned off for this form the contacts are seen as “imported” and should show up right away. In some edge cases MailChimp turns off the possibility to add contacts without double opt-in. So it’s worth testing with double opt-in turned on to see if it makes a difference.
Hope that helps. If you have any questions, please let me know!