E-Mail

If there are major issues in mail application its better to delete the account and configure it once again an article for it Set up Mail with email accounts - Apple Support

By entering in user library macOS Sierra: Library folder

Find a folder preferences > deleting com.apple.mail.plist into the trash is avoided , this can lead to either mail data loss or system can malfunction .

Second method is -

Click on Go on top menu bar , hold option key click on > library > select mail folder > right click on mail folder there itself > click on copy mail > right click on extreme right hand side in column B , that is below persistence info.plist > click on paste item .

Entire mails are copied to V5 folder .

For Mac OS High Sierra - the folder will be V5 , For Mac OS Sierra - the folder will be V4 , For El Capitan - the folder will be V3 , and for OSX Yosemite the folder will be V2 .

Go to the corrupted IMAP account > open system preferences > click on internet accounts > select the account that has to be deleted & click on minus sign .

Configure new IMAP account , go to top menu bar , file > import mail boxes > import data from > select apple mail > click on continue > select the back drop folder then choose new .

When new IMAP account is opened , a new folder with all mails will appear in the mail side bar .You can rename the folder .

The .plist preference file is located in the /Home/Library/Preferences/ folder. However, the Library folder is hidden;

Four Ways to Make the Home/Library Folder Visible

A. This method will make the folder visible permanently. Open the Terminal application in the Utilities' folder and paste the following red colored command line at the command prompt: chflags nohidden ~/Library then press RETURN.

B. Click on the Desktop, press the Option (⌥) key, select Library from the Finder's Go menu.

C. Select Go To Folder from the Finder's Go menu. Paste the following in the path field: ~/Library and click on the Go button.

D. In Sierra activate the Finder, press COMMAND-J, set Finder display to Icon view if it is in some other view mode, select your Home folder from the sidebar, check the box labeled, "Show Library Folder.

The proper way is in Mail. If the messages are in your InBox, then start there by selecting the ones you don't want then CTRL- or RIGHT-click on your selection and choose Delete from context menu. This should put them into Mail's trash. If you then go to Mail's Trash folder, you can use Select All to select all the messages in the Trash mailbox. Now in Mail's toolbar you should see a little trashcan button. Click on it to permanently delete all the selected messages in the trash.