I remember a similar issue with some newsletter software I worked on. The free version added a powered by line to newsletters among other limitations.

Unfortunately, spammers started using it which inevitably led to people thinking we were spamming them and a lot of angry emails.

The software enforced unsubscribe links but the spammers modified it to remove that. Weirdly, they left the powered by link… maybe it helped pass spam filters? Or they just didn’t care.

There wasn’t much we could do to stop them so in the end the free version had to be discontinued.