[CentOS] pop3 attack

I was looking at my maillog and it looks like someone is trying to get
into my pop3 server. 

Dec  9 15:28:54 mailserver dovecot: pop3-login: Aborted login: user=<alexis>, method=PLAIN, rip=::ffff:66.167.184.203, lip=::ffff:192.168.1.2
Dec  9 15:29:08 mailserver dovecot: pop3-login: Aborted login: user=<alfonso>, method=PLAIN, rip=::ffff:66.167.184.203, lip=::ffff:192.168.1.2
Dec  9 15:29:14 mailserver dovecot: pop3-login: Aborted login: user=<alexis>, method=PLAIN, rip=::ffff:66.167.184.203, lip=::ffff:192.168.1.2
Dec  9 15:29:18 mailserver dovecot: pop3-login: Aborted login: user=<alfonso>, method=PLAIN, rip=::ffff:66.167.184.203, lip=::ffff:192.168.1.2
Dec  9 15:29:36 mailserver dovecot: pop3-login: Aborted login: user=<alfred>, method=PLAIN, rip=::ffff:66.167.184.203, lip=::ffff:192.168.1.2

How worried should I bee about this? Any suggestions for dealing with
it?

Thanks,
James