When a dependency is missing (in this case: evolution-exchange-2.24.4), pkgdb -F will suggest an already installed port that has more or less the same name (in this case evolution-webcal-2.24.0, the name is 58% similar -- pkgdb -F is not very smart!). It is just a suggestion! If you know that the suggestion is not correct, say 'no'. Right after that, pkgdb -F will ask you if you want to install the port you really need (evolution-exchange-2.24.4). You can say 'yes' to that question. The dependency will then be installed. Mind you: sometimes pkgdb -F does get it right, and the suggestion will be correct. In that case, you can say 'yes' right away of course.