Spamkiller filters the jumk, lets you look it over without opening it or downloading it and you can either recover it or delete it, send an email adddress error so they think the email address isn't real, or even trace it back to them and send abuse complaints all the way from your ISP to their ISP and everyone who handled it...
ISP's do something everytime because spam can and will get them put on black lists...
For one thing..
If it is a webmail account, it can also be traced and addressed accordingly through your ISP's abuse link