Comcast customers: Sent e-mails blocked?

I have Comcast cable modem service here at home. It's been very reliable. Service interruptions have been rare and brief. The cable modem I was given years ago is still working fine. Network performance has been reasonably good.

So all in all, I'm happy with the service.

But I just found out about something that really bugs me--and may explain why I've received occasional reports over the years that an e-mail I sent didn't get through at all--or was marked as spam when it did arrive.

This came up about a week ago, when I noticed that some (but not all) outbound e-mails sent through Apple's .Mac service (now known as MobileMe) were not going out. After waiting about an hour for the messages to go through, I reported the problem to the service's support desk.

It turned out that the messages were being delayed within the .Mac servers by a couple of hours, and I never did get an explanation of why that happened, but the .Mac support specialist who replied to my initial report told me something that surprised me a lot.

It turns out that the IP address of my Comcast cable modem--which had been the same for several months--is in a range of addresses that is listed on several major e-mail blacklists. That is, some e-mail services regard e-mail from many (or most?) Comcast customers as more likely to be spam. This may be old news to many of you, but as I said, I found it very surprising, and I bet a lot of other Comcast customers aren't aware of it.

(This problem turns out to be unrelated to the message delays I was seeing, but it's a pretty serious problem on its own.)

As it happens, I had a brief service interruption yesterday, and in case the problem was a glitch at my end, I reset my cable modem and router. When it came back up (several minutes later, actually) I discovered I had a different IP address, one that is substantially different than the previous one.

I checked the new address against the same e-mail blacklists and found that it's on all the same lists.

You can check for yourself. The service I used to check multiple blacklists is hosted by MXToolBox; just enter any IP address--such as the address of your home Internet connection, or the address of your e-mail server--and in a few seconds, you'll get a report summarizing the responses from 123 different blacklists. (If you don't know your own IP address, try a service such as WhatIsMyIP.com.)

While the cable modem service was down, I connected via the Option 3G wireless card I use when I'm traveling. I also checked that IP address--no blacklist entries at all.

So at least I have a way to avoid this problem when I'm sending a really critical e-mail to people who aren't expecting my message and thus haven't whitelisted my e-mail address or personal domain within their antispam software.

But I don't want to have to use the Option card all the time, either.

I discussed the situation with a friend of mine who works in the network security industry, and he says the problem may be related to the proliferation of botnets on home PCs. Users click on malicious Web sites, their PCs get infected, and they start sending spam at the direction of the botnet organizers.

My friend says these infected PCs can be difficult for ISPs such as Comcast to detect. His company makes a server appliance that is designed to detect such things, so I believe it.

The bottom line here is that I have a problem I don't know how to solve. I'm sure Comcast can't solve it; it isn't in control of these blacklists, and if it could control the botnets, it would already be doing that.

I could get a different Internet service, but there aren't many choices around here, and it's quite possible that any service that has a large number of home users is going to be blacklisted for the same reasons as Comcast.

Any suggestions?