1. Dynamic, temporary/expiring email addresses. If the address includes a plain-text date of expiration, it will allow spammers to know that the address is no longer valid so don't bother sending it.
http://www.mailexpire.com/
http://www.spamgourmet.com/
e.g.
myID-20040113@somedomain.com will be good until January 13th, 2004
myID-200401@somedomain.com will be good for the entire month of January 2004.
2. Single-sender (user/domain) addresses. They allow only one sender or domain. This is similar to point 1 except that the address is a hashed string that can be used to verify that the sender is the allowed sender. This is useful for registering with mail lists.
e.g.
XDuquXQMcZ/nK+4zING8Ew@somedomain.com will only allow malda@slashdot.org emails to pass.
0jRDzvvEN4SZLpLoQu533w@somedomain.com will allow any @slashdot.org email.
Optional:
- Can be prepended with a string to indicate that the address is this type of address so don't waste bandwidth sending to it unless you know it will get through.
- The Hash can be a hash of an encrypted value to add extra protection.
3. Everyone digitally sign ALL personal/business emails and include the recipient and sender addresses within the signature . If an email doesn't contain a valid, verified signature, it doesn't pass.
These measures don't imply that you can't retain open addresses that allow all email to pass.