Because of backward compatibility we need the application to be able to compile against quite dated Qt versions.
Source codes must be able to compile against Qt-5.2.1 We need to support Qt-5.2.1 because of Ubuntu 14.10. Newly added code thus must be able to compile against Qt-5.2.1. This state is going to be preserved at least as long as Ubuntu 14.10 is going to be supported (April 2019). After this period we will probably drop this requirement. (Ubuntu 16.10 contains Qt-5.5.1.)
The project file restrict the usage to Qt-5.3 and newer. This is because we want to discourage the users to compile the application against Qt-5.2. Qt-5.2.1 contains a bug which causes the application to occasionally crash on drag&drop events.
We recommend the usage of Qt-5.4 and later. Usefull functionality was added in this release (QStorageInfo, QSslSocket::sslLibraryBuildVersionNumber()).
Qt-5.5 introduced some macros which make it easier to declare custom types to be used in the meta-type system of Qt.
Qt-5.6 added QVersionNumber, QList::constFirst(). It also added support for high-DPI scaling - although this doesn't work flawlessly here.
We compile a 32-bit application bundle with Qt-5.7.1 on OS X El Capitan. Qt-5.7.1 is the last version of Qt which compiles more or less without errors with Xcode OS X SDK 10.11 and 10.12.
We compile a 64-bit application bundle with Qt-5.9.8 on macOS Mojave. We use Qt libraries shipped within the installation package for macOS. We compile the application with SDK 10.14. Apple has announced that all new applications for macOS should be 64-bit.
We compile a 64-bit application bundle with Qt-5.9.6 on macOS High Sierra. We use Qt libraries shipped within the installation package for macOS. We compile the application with SDK 10.13.
Used Qt versions.
We use Qt-5.9.3 to compile the application bundle for Windows.
We use Qt-5.7.1 and Qt-5.9.8 to compile the application bundle for OS X/macOS. (Version Qt-5.9.6 and Qt-5.9.7 seems to cause some problems on macOS Mojave 10.14.2 where the QtSvg framework has difficulties being loaded.)
Qt versions with known problems
Qt-5.9.1, (Qt-5.10.0?) and Qt-5.10.0 display weird behaviour of the message list columns. The columns are resized when message content is downloaded. (See issue #352 (closed) .)
In Qt-5.9.3 the scaling of checkboxes and radio buttons according to the selected font enlargement (accessibility) does not work.
Qt-5.9.6 and Qt-5.11.1 have problems when connecting to records management service. The service info cannot be downloaded occasionally.