Cannot batch delete email on iOS10. The trash icon is gray, and cannot be selected.

I have an iPhone 5S that was recently upgraded to iOS10, and I use iMAP email.

Before upgrading I was able to select all messages in my inbox and delete all, however I cannot find this function after upgrading. Now when I select all messages the Trash icon is grey and cannot be selected; there is no other option available, such as 'Delete All'.

I need this option, as I am sure many other people do, since I get an excessive amount of emails per day and it is not feasible to delete them one-by-one. Currently my inbox is over 1GB.

Question:

In iOS10, is there any way to batch delete all messages in my iMAP inbox?