How can I delete 999 useless emails ..

[rolf]

In my inbox there are 999 old camera alerts now. Maximum for the folder is set to 250, but this is not my mayor problem. The question is: how can delete them all at once? 

[Kostya Vasilyev]

Unread messages?

You can long press the folder (in the account list) -> mark as read.

Or is the idea to delete these?

You'll get it done faster and easier in web mail.