Deleting an email address deletes any mailing events related to it, and the contact

I think this is critical, because it is 'data loss' of a sort.

• You have sent a contact a mailing to 'email@example.com'
• On their contact record, there is '1 Mailing' displayed
• If you edit their email to email2@example.com there is still '1 Mailing' displayed
• If you delete their email address, '0 Mailing'

The same happens if you merge two contacts, and overwrite their primary email address (which triggers a delete on the email address that is overwritten).

This is due to a foreign key constraint and 'cascade' on delete function on the 'email_id' field in the civicrm_mailing_event_queue table.

On dev we tested removing the foreign key for email_id, and after deleting the email address the 'Mailings' for the contact stayed in tact.

Though there are probably several knock-on effects of simply deleting the FK?

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Edit: This will probably happen for SMS etc with the FK on the phone_id column as well. So that should be deleted as well.