How can I clear inbox for email all at once?

To delete ALL your emails at once and if you are using the Apple Mail app, open the app, Tap Edit in the upper-right corner, tap Select All and select Delete.

Robjaxmal Said:

"How can I clear email inbox all at once?"

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Use a Computer:

If having issues with Mail on your iPhone, then use a computer, as it may be more user-friendly.