Hi kitek
You may not be able to fix it, are you running your mail server from home with your IP address supplied by your Internet Service Provider ?
If this is the case mail servers normally do a reverse look up on the domain of email addresses, so
user@yourdomain.com resolves to your IP address
Your IP address resolves to your ISP
As the two don't match mail servers treat it as forged so it's tagged as spam
I have the same problem and unfortunately I don't think there's a fix
Pete