邮箱满员退信：塞不下的附件原路返还补偿

local function sendMail(from, to, item, cnt)
    if #MAILBOX[to] >= 50 then
        return                        -- 静默丢弃，附件蒸发
    end
    table.insert(MAILBOX[to], {item = item, cnt = cnt})
end

local function sendMail(from, to, item, cnt)
    if #MAILBOX[to] >= 50 then
        giveitem(getplayerbyname(from), item, cnt, 0,
            "邮箱满员退回")           -- 原路返还
        sendmsg(getplayerbyname(from), 1, "对方邮箱已满，附件退回")
        return false
    end
    table.insert(MAILBOX[to], {item = item, cnt = cnt})
    return true
end