Depending on the settings, E-Mail messages that you deleted from within an E-Mail client like Apple Mail, MS Outlook or Mozilla Thunderbird, might be deleted only after having signed out from the client. In the meantime, those E-Mail messages will be shown as crossed out in the groupware. To delete those E-Mail messages clean up the E-Mail folder.
How to clean up an E-Mail folder:
In the folder tree, select the folder that you want to clean up.