mydomain = "example.com";
mykey = "bobble";

function ModifyReturnPath(mailfrom, origmailfrom)
    
    local srch = "@" .. string.gsub(mydomain, "%.", "%%.") .. "$";
    
    if not string.find(origmailfrom, srch) then        
        local lastAt = origmailfrom:find("[^%@]+$")
        local localPart = origmailfrom:sub(1, (lastAt - 2)) -- Returns the substring before '@' symbol
        local domainPart = origmailfrom:sub(lastAt, #origmailfrom)
        
        local timestamp = math.floor((os.time()/ (60 * 60 * 24)) % 1024);
        local timestampMod32 = Mod32(math.floor(timestamp / 32)) .. Mod32(timestamp % 32);
        
        local hash = VPOP3.Base64(VPOP3.SHA1(origmailfrom .. timestampMod32 .. mykey)):sub(1,4);
        
        return "SRS0=" .. hash .. "=" .. timestampMod32 .. "=" .. domainPart .. "=" .. localPart .. "@" .. mydomain;
    end
            print("nosub " .. origmailfrom);
end

function Mod32(x)
    return string.sub("ABCDEFGHIJKLMNOPQRSTUVWXYZ234567", x + 1, x + 1);
end