Newsletter Made Simple – Spambots hijack signup form

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Lately, on two sites with CMS 2.1.5/2.1.6 and NMS 2.12.2 installed, I’m getting a lot of “delivery failed” emails to the NMS admin email that are apparently coming from spam bots signing up for newsletters with fake/non-existing email addresses; since the confirmation emails can’t be sent an error is returned to me in the form of the “delivery failed” return emails.

There is a honeypot captcha that works but apparently the spambots somehow get around this. Is this a known/common problem? Can anything be done about this?