[Flankk]

That's really strange. Only thing I can think of is the person is using multiple throwaway email accounts to join your newsletter. They are then marking all your messages as spam in an attempt to get your email blacklisted. Hopefully someone has a less malicious explanation.

[alchemyromcom]

You know what it might be: maybe the person is a niche marketer and is going to pretend to be multiple people to sell their product to the newsletter's audience. That happens all the time. Account 1 will ask for recommendations, then account 2-6, all held by the same person, talks up the spammer's product. Although, I don't know exactly how it would work with a newsletter, I know this is very common when comment spamming.