that's part of it.
in our case, rather than infected computers sending out emails we get automated registrations, with the intent of posting links to various "performance enhancing" drugs, knock-off sportswear, etc
like most forums, heartland registrations require confirmation via email, so these spammers find free email services like gmail etc and sign up for them first in order to get a valid email address
most of this is automated (this process of automating signups is known as "scripting"), and some free email services are not as secure as others and it can be quite easy to script the signup process
recently most spammers we have had on here have used gmail addresses - it would seem that gmail's signup process is easy to script
when we ban email addreses, (i think) it only affects new users trying to register for HL with one of those addresses. existing members who have a gmail account (for example) should be ok... unless they try to change the address on their account, in which case they'll get the same "address banned" message as a new user. in this case i am happy to allow the one-off use of email addresses/domains that are otherwise banned because we know that the person wanting to use it is a real person and a valid HL member
hope that helps explain it a little
