The spam does appear to be coming in waves. It may even be that the leaked addresses were split up and sold to different people. It's also possible that after some time the known addresses will be used as the basis for more random attempts. For example, if they had the address
[email protected] they may try
[email protected]. Plenty of addresses only used internally have been leaked though.
I wish there was something more helpful I could add. If you think this address is reciving abnormal spam and do not think it was linked to the webmail platform, you could try getting in touch with one of the comms team. Although I haven't had much like on that front with an address I queried. One explanation given is that all email addresses that existed before the atmail system was implemented were imported into the system.