After an update from an older version to a MailCheck version 4.5.x or higher, the function Using MailCheck to delete unread messages is not automatically available.
To activate this function, you will have to log out and then log in again all e-mail accounts. Afterwards, the function will be fully available to you in GMX MailCheck.
To find out how to delete unread E-Mail in MailCheck using Google Chrome, click here.
To find out how to delete unread E-Mail in MailCheck using Mozilla Firefox, click here.