This link gives us a clue:
It appears that you have a script that blocks suspect accesses found in your log files. This is good, however, it may be blocking valid users too. You can find out more here: http://configserver.com/cp/csf.html
It appears from your error that you may be experiencing a brute force password cracking effort against your SMTP server (e-mail). This is bad, however, your script is protecting you.
This script may be blocking a specific IP address from a pool or blocking a domain name that represents a user pool which would block all users of that pool.
You can block the specific IP address found in your log files using firewall or iptables or whatever mechanism you have that restricts access to your server. If you have a domain name, you can use:
...to lookup the IP address.
If the domain name is something like generic-term.example.com where generic term can be something like user, customer, host, or something similar, then you may not get an IP address or at least not the proper one. This is a domain name of a pool like I suggested above. If you do not have an IP address in your log file, then blocking the pool may restrict valid accesses from that entire network. Even blocking the IP address can block accesses from a valid user. Even then, tomorrow the same thing can happen from another IP address from that pool.
There may be nothing you can do either way but to block access and suffer the consequences. It may be that you will need to keep an eye on your log files and block accesses by IP address as they occur.
Without more specific information, I am unable to give better advice except this. You want this script to do what it is doing. You may need to apologize to a few users, but at least you are not sending out thousands of spam e-mail from a hacked account.