by Jodrik » Tue Nov 10, 2009 2:13 pm
Properly implemented SPF and Use Sender Domain Check should be all that is needed to fix this.
The most common issue here is that people are not actually e-mailing you FROM your own address, simply stating that it is you / your domain and it not being true.
A novel way to prevent this is simply making a custom filter that checks wether you FROM address and the actual address used to recieve the mail is the same. This filter should check the "Header Line Matches" with the FROM address being contained somewhere in the ####FROMADDRESS####,
Note that you should test this first, for I just randomly came up with this out of the top of my head. I'd advice to start of by using this filter to set some additional HEADER tag, to see if it works for your mails for a couple of days and then use it to take action (i.e. setting SPAM flags or whatever)