Deleting a sent mailing leaves activities with invalid links to a mailing report that doesn't work
Assuming: Civicrm uses the default setting "Enable CiviMail to create activities on delivery"
If a mailing is created, sent and then deleted then the recipients of that mailing will have activities that link to a mailing report using an invalid id. If the link is followed the error will be:
Sorry, due to an error, we are unable to fulfill your request at the moment. You may want to contact your administrator or service provider with more details about what action you were performing when this occurred. Expected one Mailing but found 0
If clicked from the popup activity view it will just be a network error.
The body of the activity will also be blank after the mailing is deleted.
I am not convinced that allowing sent mailings to be deleted at all is a good design. Some kind of an archive function might be better.
In the short term however the link should probably be removed from these activities.