Digital mail. What people actually mean when they say “E-mail”.
“D-mail” is a future-proof and correct term; as the physical medium (for data transfer) is irrelevant to dmail protocols.
Consider dial-up: in “thy olden days” “email” was actually “amail” (acoustic mail). And the existence of fiber-optics turns “email” into “pmail” (photonic mail). The only constant in all cases, is that mail is encoded digitally.
You should only use the physically-specific terms when the medium actually matters and/or needs disambiguation