The recognised process for dealing with a situation like this is to send an explanation/supporting evidence to abuse(at)plus(dot)net
I can probably have a look for you though as the above method is likely to take some time.
If you can send one of the offending messages to a *non-Plusnet*
email account and then provide me with a full copy of the received email/headers, then I should be able to work out what's going on.
Send the details via PM, rather than post here of course.