is this attack or hack?

[pixel]

hello
today on suddenly all my domains are complains they are cannot send email and facing delaying for sending ,after a while of checking by run
lsof -i:25
found that allot of connections on port 25 from the below hosting . I tried to find out the IPs of those hosting to block it from my FIREWALL, i ran nslookup and dig command but the surprisingly, there are no IP address of them i cannot rich any IP address. (try to find with me) , i block hole network of qq.com and restart the smtp & qmail but still facing receiving connection from same hosting

smtpbg53.qq.com
smtpbg53.qq.com
smtpbg53.qq.com
smtpbg53.qq.com
smtpbg53.qq.com
smtpbg53.qq.com
smtpbg53.qq.com
smtpbg53.qq.com
smtpbg53.qq.com
smtpbg53.qq.com
smtpbg53.qq.com
smtpbg54.qq.com
smtpbg54.qq.com
smtpbg54.qq.com
smtpbg54.qq.com
smtpbg54.qq.com
smtpbg54.qq.com
smtpbg54.qq.com
smtpbg54.qq.com
smtpbg54.qq.com
smtpbg54.qq.com
smtpbg54.qq.com
smtpbg54.qq.com
smtpbg54.qq.com
smtpbg54.qq.com
smtpbg54.qq.com
smtpbg55.qq.com
smtpbg55.qq.com
smtpbg55.qq.com
smtpbg55.qq.com
smtpbg55.qq.com
smtpbg55.qq.com
smtpbg55.qq.com
smtpbg55.qq.com
smtpbg55.qq.com
smtpbg55.qq.com
smtpbg55.qq.com
smtpbg55.qq.com
smtpbg55.qq.com
smtpbg55.qq.com
smtpbg55.qq.com

also when i run ps ax | grep qmail i found allot of sessions for [email protected]

((my-server-name.com)) << not by domain name my hole server name these trying to send to some domains like 123.com yhaoo.com.cn
but after couple of hours the leave my server alone . but i need to know what its that
I'm still shocked and i want some explain what happened