The phrase "dubious by" is grammatically correct and can be used in written English.
It usually means something is uncertain or questionable because of a certain quality or factor. Example: His claims were dubious by his lack of evidence. In this sentence, the phrase "dubious by" highlights the reason for the uncertainty, which is the lack of evidence.
The evidence against them has been declared dubious by federal courts.
The artwork was deemed aesthetically dubious by whoever was Head of Marketing at Sire, Virgin and Phonogram at the time.
This was an explanation made dubious by Libby's own notes, which showed that he previously had learned about Plame from his boss, Cheney.
Well, his words actually pertained to fairy eggs – floating seashore objects – and also stories themselves, seen as dubious by some, and magical by others.
Why are we not made at least a little dubious by the torments, bodily and socially, suffered by Don Quixote and Sancho Panza?
Like Barthes and Bazin, Murray is a truly original thinker whose inclinations and recommendations are nonetheless rendered dubious by the sense that they're cut to fit a preconception.
These pages have a dubious by-trade in shaming cheaters.
On the other hand, the Guardian (and other news organizations) have decided to run stories based on illegally hacked emails, and no doubt will also do so using stolen financial information and other data gleaned by dubious means by intermediaries.
Flamsteed's observations had been obtained by dubious means by Edmund Halley, who was once Flamsteed's assistant at the Greenwich Observatory.
Alarmed by news reports of dubious deals by the bank's main shareholder, Tsvetan Vassilev, customers withdrew more than one-fifth of the bank's deposits.
But much of that growth, as HuffPost reported, is underwritten by dubious labor practices by the big chains that collectively employ over 100,000 people.
