I noticed several instances where copy could be updated to be more clear. These are all in the ac_newsletter.module file.

Original values:

define('AC_NEWSLETTER_DEFAULT_SUBSCRIPTION_STATUS_MSG', 'Your Subscription successful.');
define('AC_NEWSLETTER_DEFAULT_UNSUBSCRIPTION_STATUS_MSG', 'Your Unsubscription successful.');
define('AC_NEWSLETTER_DEFAULT_DOUBLE_OPT_IN_STATUS_MSG', 'Your Subscription is done. Please confirm it from your mail inbox.');
define('AC_NEWSLETTER_DEFAULT_EMPTY_ERROR_MSG_FIRSTNAME', 'You must enter your firstname');
define('AC_NEWSLETTER_DEFAULT_EMPTY_ERROR_MSG_LASTNAME', 'You must enter your lastname');

Suggested values:

define('AC_NEWSLETTER_DEFAULT_SUBSCRIPTION_STATUS_MSG', 'You have successfully subscribed.');
define('AC_NEWSLETTER_DEFAULT_UNSUBSCRIPTION_STATUS_MSG', 'You have successfully unsubscribed.');
define('AC_NEWSLETTER_DEFAULT_DOUBLE_OPT_IN_STATUS_MSG', 'Your subscription is complete. Please confirm by checking your email.');
define('AC_NEWSLETTER_DEFAULT_EMPTY_ERROR_MSG_FIRSTNAME', 'You must enter your first name.');
define('AC_NEWSLETTER_DEFAULT_EMPTY_ERROR_MSG_LASTNAME', 'You must enter your last name.');