Hi Room101A,
Thanks for coming to Apple Support Communities for help today.
From what you have described, the account on the Mac is behaving like a POP account. Which email provider does she use? Check the webmail portal through Safari and see if the emails are deleting there.
If email isn't deleting there when deleted on the Mac, then the issue points to how the email account is set up on your Mac. If that's the case, try removing the account on the Mac and setting it up again.
Please see: Log out of or remove email accounts in Mail on Mac.
"You can remove an email account from the Mail app on your Mac at any time. When you do, the account’s email messages are removed from your Mac. Copies of messages remain on the account’s mail server and are still available (from webmail, for example).
Important: If you’re unsure whether messages you want to keep are still on the mail server, move or copy them to a mailbox stored on your Mac (the mailbox appears in the On My Mac section in the Mail sidebar) before you remove the account in Mail.
- In the Mail app
on your Mac, choose Mail > Settings, then click Accounts. - Select an account, then click the Remove button
.
Note: If the account is used by other apps on your Mac, you’re asked to remove the account in Internet Accounts settings. Click the button to open Internet Accounts, then turn Mail off for the account. To stop using the account in all apps, make sure the account is selected, then click the Remove button."
Then, you can follow these steps to add the account back: Add email accounts in Mail on Mac.
If the issue continues after this, we'd recommend contacting the email service provider for more information.
Have a fantastic day!
