Columba is free software that is licensed under the Mozilla Public License. This license allows you to use, modify and redistribute this software. There's a nice FAQ which should answer most of your questions. Note, that we have choosen this license intentially to become more business-friendly. There's no virual effect as seen with the GPL here.
Columba is not a monolithic application. It makes use of dynamically loaded extensions. These extensions are what we call plugins. The MPL License lets you create closed-source or binary plugins and allows you to sell these without a problem.