How hard.....or easy would it be for someone to delete an email days after they had sent it to you?

In conventional way, If the service sending mail and receiving the mail are same/integrated, and the service provider provides the option, it's possible. Otherwise if the mail has been delivered to recipient mail server, sender can't do anything.

But that was a thing of past, have a look at it.

https://www.raymond.cc/blog/message-or-text-that-automatic-deletes-after-viewing/

https://www.cnbc.com/2018/08/20/how-to-send-self-destructing-email-gmail-confidential-mode.html