We're experiencing a strange situation with our client's e-mail accounts, in that they are being intermittently blocked by spamcop because the addresses 'do not exist'. A popular ISP in the area is using the reputable spamcop software, and I was wondering how spamcop determines that addresses might not exist (when they clearly do). Here is the failure response message that the ISP's outbound server generates: The following message to <webtech[at]emmatt.com> was undeliverable. The reason for the problem: 5.1.0 - Unknown address error 550-'found in bl.spamcop.net' Reporting-MTA: dns; fipsb03.cogeco.net Final-Recipient: rfc822;webtech[at]emmatt.com Action: failed Status: 5.0.0 (permanent failure) Remote-MTA: dns; [18.104.22.168] Diagnostic-Code: smtp; 5.1.0 - Unknown address error 550-'found in bl.spamcop.net' (delivery attempts: 0) Our mail servers are primarily at mail.emmatt.com, on a shared server. The address listed is my own, and definitely exists... however, this happens on numerous accounts under the same hosting server, with different domains. Everyone who uses this popular ISP reports this intermittent issue when trying to contact us or our clients. Could someone shed some light on this? Thanks in advance.