Take stock of what's running
lsof -i :25
lsof -i :465
lsof -i :587
lsof -i :110
lsof -i :143
lsof -i :993
lsof -i :995
Also allow your STARTTLS and SSL/TLS ports in CSF (/etc/csf/csf.conf):
TCP_IN = "20,21,22,25,53,80,110,143,443,465,587,993,995,2031,2083,2087,2096,2304"
PORTS_imapd = "143,993"
PORTS_pop3d = "110,995"
SMTP_PORTS = "25,465,587"
PORTS_smtpauth = "25,465,587"
PORTS_eximsyntax = "25,465,587"