Blacklisting

We had a problem sending e-mail from an Outlook account on one of our laptops last week and discovered that the problem was due to our IP address being blacklisted by one of the Blacklist services used by our ISP UPC. The problem was due to a customer's PC that was in the workshop which was infected by spamming software and which we connected to our network. Once you are blacklisted you have to make a delisting request directly to the Blacklist service provider, not to your ISP (UPC in our case). This can take up to 24 hours before it happens, leaving one without outgoing e-mail on SMTP-based mail clients e.g. Outlook for that period (Gmail and other web-based mail is not affected).

This is the second time that this has happened since we moved to UPC - it never happened us during the 12 years we were with Eircom - perhaps Eircom don't utilize Blacklist services.

Anyway, all is well again and to prevent a recurrence we have blocked port 25 on our router for all IP addresses except the one assigned to our PC using Outlook. Any customer's computer that is infected with spamming malware will now be blocked from sending via SMTP on the blocked port.

Here is the description of blacklists from www.blacklist.ie where you can check your own blacklisting status.

Blacklists are created and managed by anti-spam organisations and are seen as an effective way of punishing known spammers.
Some of the most widely checked blacklists are Spamcop, MAPS blacklist, and SPAMHaus - blacklist.ie checks against these and many more. These organisations publish blacklists of IP addresses that are known spammers. Blacklists are used by thousands of ISPs as part of their defense against spam. A legitimate email system can also end up being blacklisted, resulting in mail delivery problems and subsequent costs for business.
Internet Service Providers (ISPs) subscribe to blacklists in order to filter out spam sent across their network or to their subscribers. If you are listed and an ISP checks against the list, your message will not be delivered.