When you read a message to your iPhone ( or iPad or iPod Touch) inside the mail application, in iOS default settings, your program will provide you with an archiving function , you can remove the message you read from your Gmail inbox in this way .
Your information will be removed automatically and all be saved in the All Mail folder.
This will give you unnecessary trouble , you will have to face a lot of mail’s cleanup work in future someday .
If, after you read the message , you can delete it directly , it would be a good idea .
If you do not want the default archiving functions and wanna how to set up a delete option?
The following is the detailed steps of how to delete your iPhone Email:
First, open the Settings, and then click “Mail” > “Contacts” > “Calendar”, then select the email account you want to change. You will see an option at this time, says to archive messages. Turn it off , and now you can send a message straight to the trash folder from the Inbox.
If your mail application associated with a Gmail account, the above method is functioning. If you have configured the Yahoo! Mail on your iPhone, it will be no way to change the default status of iOS and providing the Delete option.
By the way, the iPhone does not support delete emails in bulk way.