Email goes always to spam

==== Required information ====
- iRedMail version: v0.3
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): LDAP
- Linux/BSD distribution name and version: Ubuntu 12.04 LTS
- Related log if you're reporting an issue:
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Hi, I'm new using this iRedMail and i know it is a good mail server, i'm having a problem about my emails because i notice that when i'm sending emails to my client the email goes always to spam.. is there any tweak or tutorials on how to fix this.

Thanks in advance