General :: Secure Box - Authentication Failure - Long Strong Password Can Stop To Prevent From Attacks?

we are using linux email server axigen past few years. we keep port open ssh and pop,smtp webmail etc. ssh use for remote trouble shooting. so through firewall it is globally accessable. we notice many attacks coming to our machine, also some people try to enter in our system but failure. as example see below a log come in messages file

Mar 17 09:19:50 sa1 sshd(pam_unix)[21231]: authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=210.51.13.120 user=root how we can secure more. as per my understanding only good long strong password can stop to prevent from attacks.