I have this problem, i have a remote mikrotik connected to ISP with static ip’s, the ISP does not allow smtp relaying over the connection, so created a l2tp vpn to my server just for mail, i need to switch of natting, but as soon as i switch of natting (masq) email works and no http, if i switch natting on again they both work then mail stops with error limit exceeded due to all mails from that router is now coming from 1 ip, so how can n redirect all other traffic through the ISP side and switch off natting? 
I guess you have to turn off “Add default route” in tunnel settings.
It seems to me, that, when tunel is established it sets defautl route to itself so all trafic is forwarded through tunel. When defautl route is turned off, then only traffic meant for tunnel (determined bu IP range) goes through VPN, an everything else goes to other default route, which should be connection to your ISP