I run a mailing list and over the last few days someone has started automated subscriptions with non-existant email addresses.
These all originate from different IPs and the subscriptions are to non-existant domains (just random characters).
Considering that this has no purpose because the email accounts can't be activated or receive mailings, can anyone think why on earth would someone bother to do this? What's to gain for the spammer other than wasting a few minutes of my time?
often these are spambots that probe your site by submitting forms with unique strings for the form inputs to see if the content gets published. later there may be additional probes to see if urls get published as anchor links and whether or not they are nofollow. eventually the goal is to publish spammy links on vulnerable UGC sites or in comment spam.
the spambot doesn't "know" it's a subscription form.
Going through a service like feedburner(owned by Google) may be of help since it regularly updates and monitors their anti-spam capabilities. They have an option to sign up for new posts by email still and you can set up the feed to display specific content.
Also, subscribing to the email feed on some sites also gets you an account to post comments with, without verifying the email address, and the bots on your site may be looking for that in order to spam their links.