If the spam was directly to you and not through the list, then the reason
is that the spammers search through webpages for email addresses, and since
most mailing lists have a web-archive, you really cannot expect your email
address to remain private after you post to a mailing list.

Stopping mail going indirectly through the mailing list should be easy thou,
using tools like spamassassin.

-Frank.

> From: Seth, Wayne (Contractor) [mailto:sethw AT fhu.disa DOT mil] 
> Sent: Wednesday, March 19, 2003 6:09 PM
> To: 'amanda-users AT amanda DOT org'
> Subject: Pornography
> 
> Since signing up on the Amanda list serve about 2 or 3 weeks 
> ago, my e-mail address seems to have distributed to several 
> pornographic spammers.  Because of that, I cannot continue 
> with the list.  Please unsubscribe me.