
What do you do if you wish to delete emails to de-clutter your Gmail account, but you’d like to keep a copy of some emails in case you need them in the future? If you use Gmail in the Apple mail app, and you delete an email in Gmail, it’s deleted from your computer as well. There is, however, a way to keep a copy of the email on your mac before you delete it, in case you ever need to access it. 
There’s a drop down menu called “On My Mac” in the sidebar in the mail app. If it’s not already dropped down, you will need to click the ‘show’ button that appears when you hover the mouse over it.

All you need to do is select the email or emails you want to keep and drag them into one of the folders underneath the “On My Mac” heading. This will make a permanent copy of the email from Gmail to your computer’s hard drive.
Now you can delete the email/s from your inbox, but you will still have a copy of them in case you need them later.
How do you do this in the gmail app for iPhone?