Are you allowing outgoing mail ports in /etc/csf/csf.conf?
TCP_IN = "21,22,25,53,80,110,143,443,465,587,993,995,2083,2096,2304"
TCP_OUT = "20,21,22,25,22,53,80,110,113,443,2083,2096,587,993,995,2080,2443"
SMTP_PORTS = "25,465,587"
PORTS_imapd = "143,993"
PORTS_pop3d = "110,995"