I got unsubscribed from a mailing list this week.
The reason was that the mailing list forwarded me a virus. Several times. My MTA rejects viruses at SMTP time, but the mailing list software took this to mean my address was broken and promptly unsubscribed me.
Conclusion: All mailing lists should have virus protection regardless of what platform they expect their readers to be on. Anything less is irresponsible.