Check at server
[root@dedi25 ~]# telnet mx1.mail.sg1.yahoo.com 25
Connected to mx1.mail.sg1.yahoo.com (126.96.36.199).
Escape character is '^]'.
5.7.1 [BL21] Connections will not be accepted from 188.8.131.52,
because the ip is in Spamhaus's list; see
Connection closed by foreign host.
Remove IP Spamhaus