  The enhancement in the dilepton spectrum observed in heavy-ion collisions for
invariant electron-positron masses in the range 0.15 GeV/c$^2 < M_{e^+e^-} <$
0.6 GeV/c$^2$ has recently been traced back to a corresponding enhancement in
$pn$ collisions relative to $pp$ collisions. Whereas the dilepton spectra in
the latter are understood quantitatively, theoretical descriptions fail to
describe the much higher dilepton rate in $pn$ collisions, in particular
regarding the region $M_{e^+e^-} >$ 0.3 GeV/c$^2$ at beam energies below 2 GeV.
We show that the missing strength can be attributed to the $\rho$-channel
$\pi^+\pi^-$-production, which is dominated by the $t$-channel $\Delta\Delta$
excitation and the recently found isoscalar dibaryonic resonance structure at
2.37 GeV.
