If you are receiving the error:
Error code: ssl_error_rx_malformed_server_key_exch
Try deleting the cert.db file in your profile directory. Here's how:
1) Quit or Exit from Postbox so that it is no longer running.
2) Navigate to:
Note: In macOS the Library folder is hidden. To access it, open the Finder, hold the Option key, then select Library from the Go menu.
Note: The AppData folder is folder is a hidden folder; to show hidden folders:
Windows Vista / 7 - Open a Windows Explorer window and choose "Organize → Folder and Search Options → Folder Options → View (tab) → Show hidden files and folders".
Windows 8 and higher: Open a Windows Explorer window and choose "View Tab → Options → Change Folder and Search Options → View Tab → Show hidden files and folders, then click the Apply button".
3) Delete the cert.db file.
4) Start Postbox.