In Dutch dodelijk (and its variant dodelijke) does mean mortal, but indicating causing death, fatal, as in a mortal wound (Dutch: een dodelijke wond). The meaning susceptible to death (#1) should be translated with sterfelijk (and its variant sterfelijke).
mortal enemy, mortal foe
We don't have a definition for this. I was gonna enter the Middle English word mortel but I couldn't figure out what gloss to use. I look here, and we don't have an entry for it anyway. A mortal enemy isn't susceptible to death (well, not more than the rest of us). Mglovesfun (talk) 14:34, 19 August 2010 (UTC)
Mortal as causer of death
Mortal wounds, mortal fear, etc, don't fit the first definition; this sense seems almost (but not) synonymous with "lethal".