I have--after several go-arounds with brain-dead support people--finally gotten my ISP to state directly:
"We are blocking the error messages ("bounce-back") sent when you attempt to send e-mail with a bad name, address, etc."

I find this incredible. I am now on my 5th ISP in less that ten years, and NOONE has had this policy.

Does anyone know of other ISPs that have this policy?

One has to wonder why you are sending email with bad delivery information.

I do not intentionally send e-mail with bad addresses!!
The issue came up when we sent e-mail with a typo in the address---an got no "bounce-back" message (The way one does on all other ISPs)

Obviously, if one never makes a typing error, this would not be an issue......

Are you saying that this happened with the very first incorrectly addressed email? I don't know if my ISP does this. It seems to be a bit extreme.

I use Comcast and I'm located in central Massachusetts, USA. I know that they will disable sending emails if a particular address shows a lot of activity of sending emails. I found out about this when I changed the NIC in my "Internet" computer at home. I got a new DHCP address. It happens that the new DHCP address had previously been blacklisted by Comcast as a mail spammer. I could still use their web interface to my email but I couldn't send email from my computer for about a week.

Anyway, I only mention that as my idea of a reasonable policy. Wait for a history of infraction then take evasive measures. If your ISP's default policy is to never deliver bounced email then that seems to me to be overkill. Oh well. I guess we might think differently if we were managing that ISP.

After several mind-numbing episodes with the tech support people, I ran a series of tests. The results were consistent: I found about 5 domains where error messages consistently were returned** using my office e-mail. Using various combos of POP client and OS, NONE of the error messages were returned when using the Adelphia account.
There was one curiousity: For ONE combination using Adelphia, a test message to the domain which started to whole issue was returned. Is it possible that they patched just that one to try and placate me??
I have posted this on Broadband Reports and--so far--no reports of any other ISP doing this.

**It appears that many domains do not send back an error message--eg AOL. No extensive study on this aspect, however.

Um, you mean Comcast blacklisted an IP address from their own DHCP pool? Man, that just might be about the dumbest thing I've ever heard of them doing.

I think it was a good idea. Imagine that a pc gets infected with a spam bot. This would help to identify that problem. When the person with the spam botted pc calls in to tech support they will be told that they should run a virus check. I think it's a good idea. The blacklisting only lasts for about a week if you do nothing. I didn't even talk to their tech support. I found out about the black listing on their FAQ page. I completely support this approach.

You missed my point....They blacklisted an IP address and then put it back in the pool without removing it from the blacklist. That guarentees a tech support call from someone who is now blacklisted for no discernable reason. Much time and effort spent by both Comcast and the user trying to solve a problem that doesn't exist. In the meantime, the spam bot may, or may not, have been cleaned up but it has moved onto a new IP address.

I agree, blacklist them if they are are suspected to be infected, but do it intelligently. Comcast can identify individual cable modems. That strikes me as a MUCH better approach to blacklisting since users rarely swap cable modems.

I have finally gotten Adelphia to admit that they are purposely blocking "bounceback" error messages. I have informed them that, in my opinion, they have no right to do this. (No more than they have the right to block e-mail from my mother).
They are now researching to see where their policy is documented and --I infer--checking to see if this is really what they want to do.
Meanwhile, I have posed the question here and at Broadband Reports--So far I find NO precedence for Adelphia's action.

I will report any news--meanwhile would like to hear from anyone with relevant experiences.

-Thanks