The email account gets synced to the mail server if you have configured it using IMAP. To delete emails from the IMAP delete email server, carry out the step-by-step instructions provided here.
If you delete the IMAP delete email from your email account, the emails will be deleted from the email server if you are using IMAP incoming server protocol.
If you want to stop deleting emails from the mail server, change IMAP to POP and then perform the instructions provided below.
If you are not able to delete the emails by selecting the Delete option, follow the instructions below.