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
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