There is only one MaxBulk Mailer software. It is the product you can download here. MaxBulk Mailer can be registered as Standard or as Pro though. It all depends on the serial number/activation code you have purchased:
A standard serial number activates the software and removes all the trial version limitations such as the unregistered footer added to your outgoing messages and the deliveries per session restriction.
A Pro serial number go a step further by also activating a few advanced features. You will find the full list of those Pro exclusive features here.
You can upgrade from MaxBulk Mailer Standard to MaxBulk Mailer Pro at any time here making sure you select the reference that applies to the version you currently have. You will receive a new serial number to convert your Standard version into a Pro version. Use the 'MaxBulk Mailer > Upgrade to Pro Version' menu on Mac OS X or the 'Help > Upgrade to Pro Version' on Windows to enter your new serial. More information on this here.
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