I've used this for years to help cull my email and never thought of sharing it until now.
Select a message in Mail.app and trigger this macro to find all messages from selected sender in all inboxes. For example, when I have a bunch of daily newsletters backed up as I am processing my inbox. I'll select one, fire this macro, delete.
I have it limited to my inboxes, but you can modify it to suit - delete the last action if you want to see all messages in all mailboxes.