Last week, Demon customers discovered that they could not send e-mails to recipients with Excite accounts.
An Excite spokesman said, "We are blocking all of Demon due to a violation of our e-mail guidelines."
He would not specify which guidelines had been broken or by whom.
The decision follows problems where spammers used Demon accounts - a result of Demon's policy of allowing customers to host open mail relay on their e-mail servers. However, in this case, no evidence was produced to suggest that the e-mail accounts affected had been misused.
A Demon spokesman said, "Excite blocked a range of IP addresses. It is difficult to say what was going on - Excite gave no more information - but it was probably an automated response to perceived misuse by spam e-mailers. This type of situation is a problem for the industry. It is a blunt instrument - it would be easier to exchange information to resolve such matters."
Tony Lock, senior analyst at Bloor Research, said, "Surely it is better that one ISP talks to another to sort out problems like this rather than take it out on the customer. It could be interpreted as an anti-competitive measure."
Demon customer Michael Ricks, a Sussex business consultant, said, "This situation has been going on for months and I have had the distinct impression that someone has been economical with the truth. I have had conflicting stories and no real answer despite repeated requests and complaints. If I pay for a service then I expect to be able to send and receive e-mails."
Excite was not available for further comment.