Why am I selling penis-enlargement pills?

I received a message on Facebook today from a long-standing friend who was concerned that her Hotmail email account had been hacked, because people were using it to send spam. Quite reasonably, she was wondering how this could have happened. After I sent her a response, it occurred to me that other people might find it useful and so I’m posting a slightly edited version here – the other two blogposts I have gestating can wait for another day. I intend this to be comprehensible to non-techies so I will gloss over some of the details.