Hey there muusi,
If you have not had a chance to check out the information on this page, please do so: Organize email in Mail on iPhone. It provides information on how to edit the mail in the inboxes of your Mail app.
1. While viewing a list of emails, tap Edit.
2. Select the emails you want to move or mark by tapping their checkboxes.
Tip: To select multiple emails quickly, swipe down through the checkboxes.
3. Choose the action you want to perform on all the selected emails.
If you're not using the built-in Mail app, what app are you using? Have you reached out to the developer of the app to see if they can assist with this issue? Here's a guide to help: How to contact an app developer.
We hope this information helps you out.
Kind regards.