[wlug] mail servers

can someone please recommend a lightweight open source mail server. I'm looking at connecting up my old android phone to a solar charger and running services from it, so whatever i use needs to be small. the phone i have to work with that has 512mb ram. android itself uses around 220-230mb of ram, thats before i go silly on rooting it and removing stuff, not that theres much to remove. -- Kind Regards Rowan Schischka