[Justsmile]

tmp = strtok(cmdtext, idx);
if(!strlen(tmp))
{
SendClientMessage(playerid, COLOR_WHITE, "USAGE: /emails [playerid/PartOfName] [text]");
return 1;
}
new Ereceiver;
Ereceiver = ReturnUser(tmp);
if(Ereceiver != INVALID_PLAYER_ID)
{
PlayerInfo[playerid][pAccountMailSendTextCreate] = 1;
PlayerInfo[playerid][pMailReceiver] = Ereceiver;

is this right to save the ID of the "receiver"?
Yes, if the ID left it wont work, but if the two (playerid, ereceiver are online.