Hi Michael. I did an IP whois lookup on the range of IPs in question and I see they belong to a company in Seattle called ServerCrate. I went ahead and contacted them to let them know that someone is using those IPs to send viruses and spam. Hopefully they will shut them down. I have seen this problem many times in the past. Normally it's alright to blacklist an entire range of spamming IPs if you see that they come from another country. However, this range is in the U.S, so I can understand why you asked the question about blacklisting valid servers. Unfortunately, there is no way to stop a spammer or bot from physically doing this. Your best bet is to blacklist the range for now and if any valid mail tries to come from those IPs and can't get through, I'm sure one of your customers will report it to you and you can reassess at that time. Also, as you surely know, it's best to have several layers of virus, malware and spam protection for your server and network. I hope this info helps.
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