I've discovered a couple of other issues while clearing down the 200000+ of the same email.
All actions were done in SmarterMail interface:
1) When you do a search on that name you are only returned around 20000 of those results (I know this because once you've deleted those items you are presented with another 20000 ish)
2) When you delete that amount of items, it shows the system deleting them fairly quickly but then looks like it hangs, but it isn't hanging, it then indexes that mailbox and then optimises that mailbox, this takes longer than the delete itself but there's no notification that thats what its doing.
3) Once i'd cleared the emails I was being returned from the search (I thought i'd completed the task), when I then viewed without the search and discovered another 35000 that weren't being returned in the search and could only be viewed without searching.
So I think there's an issue with the search not being able to find all the items in that mailbox
There also seems to be a limit to the returned results.
The system could do with notifying the user that the mailbox is indexing and then optimising that mailbox just so you don't try and do other bulk processes or restart it because you think its locked up.