Go to iPhone Settings -> Accounts & Passwords -> your email account -> Advanced -> INCOMING SETTINGS -> Delete from server, you can check NEVER, SEVEN DAYS, or WHEN REMOVED FROM INBOX, just choose WHEN REMOVED FROM INBOX. Then when you delete a email on iPhone, it will delete on email server.
Accessing your email via an email client (like Thunderbird) will have similar behavior: deleting an email will usually just mark the email for deletion and it won't truly be deleted (or expunged) from the your storage on the server until some set amount of time later (such as two weeks, set in your preferences).
Go to iPhone Settings -> Accounts & Passwords -> your email account -> Advanced -> INCOMING SETTINGS -> Delete from server, you can check NEVER, SEVEN DAYS, or WHEN REMOVED FROM INBOX, just choose WHEN REMOVED FROM INBOX.
Tap 'Delete from server' and choose 'Never. ' This means you can receive emails on your iPhone, but doing so doesn't delete them from the server. Review your email provider's POP settings and use them to set the correct Server Port or turn on SSL, if it's required.
If you've permanently deleted an item in Microsoft Outlook or Outlook on the web (formerly known as Outlook Web App), the item is moved to a folder (Recoverable Items > Deletions) and kept there for 14 days, by default. You can change how long items are kept, up to a maximum of 30 days.
By default, email accounts on iPhone try to use IMAP whenever they can.What settings do I need to change on my iPhone?
A POP3 accounts only downloads email from the server on a one-time one-way basis. If the option for "Leave messages on server" is not checked, the items downloaded are then immediately deleted from the server. For a POP account, anything deleted on the desktop is never removed from the server.
By default, when you retrieve new messages from a POP3 email account, the messages are deleted from the POP3 mail server and then saved on the computer that you are using. Because the message is no longer on the POP3 mail server, you can't see it when you connect from another computer.
Select your mailbox and click on “change”. Click in the bottom right on “More settings” and go to the “Advanced” tab. Under “Delivery”, check the options “Leave a copy of messages on the server” and “Remove from server after removal from 'deleted items'”.
Updating Mail Servers on iPhone
Fix for “this message has not been downloaded from the server error.”
IMAP synchronization will delete the server copies too. If your account connects using POP, then deleting server copies when you download messages is the default behaviour, unless you specifically change it by setting the checkbox "Leave a copy on server".