We report on the search for anti-neutrinos of yet unknown origin with the Borexino detector at the Laboratori Nazionali del Gran Sasso. In particular, a hypothetical anti-neutrino flux from the Sun is investigated. Anti-neutrinos are detected through the neutron inverse beta decay reaction in a large liquid organic scintillator target. We set a new upper limit for a hypothetical solar (v) over bare flux of 760 cm(-2) s(-1), obtained assuming an undistorted solar (8)B energy spectrum. This corresponds to a limit on the transition probability of solar neutrinos to anti-neutrinos of 1.3 x 10(-4) (90% C.L.) for E ((v) over bar) > 1.8 MeV, covering the entire (8)B spectrum. Best differential limits on anti-neutrino fluxes from unknown sources are also obtained between the detection energy threshold of 1.8 MeV and 17.8 MeV with more than 2 years of data. (C) 2010 Elsevier B.V. All rights reserved.