When changed my ISP, outbound e-mail stopped working, which I expected, for reasons that I did not.

I could have sworn that all my outgoing e-mail was being relayed via a server at my old ISP. This turns out not to have been that case. Specifically, I could find no trace of any reference to any ISP smtp server.

I have to assume that the old ISP was not stopping my system delivering e-mail directly. The new ISP quite definitely does.

So, I added an entry to my /etc/postfix/main.cf file as per http://nlug.org/mail/nlug__2004_01/0177.html. This re-enabled my outbound e-mail.