Terry Zink has been posting articles about botnets as traced by Hotmail. I do not often talk about botnets as they are outside my area of expertise. They are not something I deal with, as no one who uses botnets is welcome as a client here.
My clients and I, however, do have to deal with the fallout from botnets. Because of botnets, receiver ISPs are extremely suspicious of mail from any IP address that they have not seen mail from previously. Mail from new IPs is, more often than not, a newly infected Windows machine. This results in mail from new IPs not starting with a reputation of zero but starting with a negative reputation.
Botnets are another example of spammers making it more difficult for mailers with permission to use email.