MaxBulk Mailer can be activated/registered as Standard or Pro, depending (as explained here) on the serial number you purchase. The Pro version adds the following exclusive features:

• Styled text format support - Bold, Italic, Underline, Color, Font, Size...
• Conditional message contents depending on optional tags value.
• 20 Custom Tags for advanced mail merge.
• Ability to process tags in subject.
• 13 date tags, Long Date, Short Date, Abbreviated Date, Long Time, Short Time...
• Quick selector in order to check/uncheck recipients depending on their values.
• Recipients creation directly from Filemaker. (Mac OS X only)
• Hyperlink manager to insert text and HTML hyperlinks tags with one click.
• Full Applescript support. (Mac OS X only)
• Full mySQL, postgreSQL and OBDC database support.
• Secure SSL connection support.
• Possibility to use several server at once.
• Cc and Bcc support.
• Message opening and click-through tracking.

You can upgrade from Standard to Pro at any time. You just need to purchase a Standard to Pro upgrade serial 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.