Deleting emails simultaneously across all devices?

Mail messages are not directly synchronised between devices - but instead individually synchronise with your Mail Provider’s mail server.

If mail clients are using an appropriate mail protocol, such as IMAP, the mail status (read, unread, flagged, deleted or moved) is synchronised with the Mail Server; all mail clients that access the same mail account will see the status changes and update to reflect the changes.

As such, when you delete you local copy of an email on one device, the deletion also occurs at the Mail server - and the deletion is replicated to other clients that access the same mail account.

If instead you are using the older POP3 mail protocol, you are likely to see completely different behaviour.

You have to have an IMAP email account in order to do this. In an IMAP account, actions are reflected across all your devices. This is not possible with POP email.