Log Message:
Update to 2.3.0beta5:

* When new mails arrive, the tray icon is changed, and it will
  be kept until a new mail is read.
* The option "Open inbox on startup" was added.
* The option "Remeber last selected message" was added.
* The crash when printing a message with empty subject was fixed.
* The bug that the text in the composition window was not redrawn
  on the undo of large text was fixed.
* POP before SMTP was implemented.