I manage some mail servers for an ISP. There have been times when I've came across domains that we still have control over but the companies are long gone. Many times I'll "recycle" domains (old ones of mine, "donated" domains, or some of these "forgotten" domains) and use them for spam traps.
Typically, I wait until a domain hasn't been used legitimately for e-mail for at least one year (i.e. I'll set "null" MX records or point them to a non-existent host) before I repurpose them. You might be surprised at the types of e-mails that start flowing right back in after pointing the MX RR back to a real host: lots of it is crap (spam), but I've received travel itineraries, notifications from AmEx, appointment reminders from medical facilities, and all kinds of good stuff.
Typically, I wait until a domain hasn't been used legitimately for e-mail for at least one year (i.e. I'll set "null" MX records or point them to a non-existent host) before I repurpose them. You might be surprised at the types of e-mails that start flowing right back in after pointing the MX RR back to a real host: lots of it is crap (spam), but I've received travel itineraries, notifications from AmEx, appointment reminders from medical facilities, and all kinds of good stuff.