If you must know, the message said:
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Hello <author>,
I am contacting you to offer to sponsor your addon <addon>. We have recognized how useful your addon is and how much time and work goes into creating and updating it. We currently advise all the members who purchase our <Blah> to download and use <addon>.
We are offering to pay you every time you update your addon (our company pulls in a good amount of revenue so we can afford to pay you well). All you have to do is make it so when someone logs in for the first time after updating your addon a window will appear stating that <addon> is sponsored by <Blah2> and have one or two lines of text about our <Blah> with a link to our website. This only has to show up the first time you login after an update NOT every time they login.
The sponsorship window was just an idea if you don’t want to do that and have another suggestion I am willing to negotiate. If you are interested at all please email me back with any terms or conditions you might have and also how much you would want to get paid for this sponsorship.
<Blah's website address>
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It quite clearly violates the addon policy as it is paid advertising, and thus I applaud the quick action to delete the messages - however, email copies of the message (wowi sends emails on PMs received with the contents) do remain.
Can you imagine if 10 addons you use suddenly popped up a box after patch day? Disastrous.