The problem…

clients of certain isp cannot connect to my sites

Have any of you yet had the problem where no one from a particular isp could:
1. see any of your websites
2. engage in any email via your mail server

(this includes clients of htat isp as well as workers there)
I've got one such problem with prexar.com - In fact, I challenge any of you to get an email thru to anyone there. [support @ prexar.com] is valid if you want to try. I've been going back and forth for 2 months now and Manny is currently on the job trying to finally track it down but I thought I'd check in the forum to see if anyone else had ever had this happen. We know what it's NOT

1. nameservers are squeaky clean
2. our ips are not blocked at prexar
3. prexars ips are not "obviously" blocked at hostdime
4. prexar does not subscribe to any spam lists which would cause this.

Have you contacted this particular ISP? It sounds strange but could be a misconfigered proxy at their end.

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Sounds like a vrute force firewall issue to me.

It could be also problems with network connection - some isp's have peering problems so they may have no connection for certain websites.