CEAS 2008 is doing another Spam Challenge test of various spam-filters, and as part of this, they need samples of ham mail messages.
As part of the data collection effort, we have set up a website through which it is possible to donate non-sensitive legitimate email, to be used in the evaluation. Any kind of email that the recipient considers legitimate is welcome, including computer generated (non-spam) messages.
After the CEAS evaluation, the benchmark data will be made publicly available to facilitate future reasearch and development in the field of spam prevention.
Here is the collection site; they accept UNIX mbox format, and tar.gz or zip files of same, with an 8MB upload limit.