This is not the same problem but very similar and should give you a kickstart.
This is Evren Yurtesen's respons to a guy, taken from my email archive:
Bill Wheeler wrote:
> Greetings! YazzY on #RouterOS gave me your name and told me to contact
> you regarding a problem I'm having. I'll try to give you all the
> details I can about the issue.
> I have many clients behind the router in 10.0.x.0/24 networks. I have
> two outbound connections, SDSL & Cable. We use the Cable as a backup
> just in case of problems usually. Well the SDSL provider has decided to
> block all outbound port 25 traffic that's not destined for their SMTP
> server. As I don't want to change over 200 PC's mail servers, I'm
> forced to come up with another solution.
> What I want to do is send just the SMTP & POP3 traffic out the Cable
> connection but leave everything else on the SDSL. I've tried src-nat,
> but it just refuses to work. I have tried it with a single interface
> for both outbound connections and with an interface for each. The
> traffic just does not obey my src-nat rules.
> YazzY has setup this type of thing with his network, but he did it with
> two IPs from the same provider on the same line. So I'm not sure it was
> identical or not.
> I've tried setting the to-src-address and leaving it blank. I've tried
> everything I can think of with no luck. The only time I was able to
> make it work was with ICMP only. I could traceroute out the alternate
> network, but can't seem to get any TCP traffic to use the alternate
> If there's any way you could help me with this, I'd be very grateful.
> If you have any sort of IM or IRC client and would care to chat
> real-time, I'd be more than willing to at any time of the day. I look
> forward to your reply.
You should have multiple routing tables in your mikrotik and use
flow marks to define which packets will use which table...
First from firewall, you go to Mangle section (well if you are using
winbox) and make a mangle rule select protocol TCP and port 25
and you should select in Action tab the action as Passhtrough and
FlowMark as hmm SMTP, it can be an text really, but this will mark all
your smtp traffic so we can reroute this later. At this moment this
setting wont effect anything until we make the changes in the routing table.
Then you go to IP-> Routes and there you add another table at Tables
tab. Lets name it as Cable and the main default route probably shows the
Then you go to Rules table and there you select Flow as SMTP and Table
as as Cable. This will make sure that the SMTP flow will be routed using
the Cable routing table.
You go to Routes tab now and from the dropdown list be sure that the
Cable routing table is selected. (There should be 'main' table also).
In the cable routing table, you just put a default route with
destination 0.0.0.0/0 to the gw IP address of your cable router.
This should make all the traffic to port 25 of any host going through
your mikrotik to be forwarded to your cable gateway's IP address. Then
the gateway would forward it out.
I think its pretty simple way to do stuff. Next if you wanna add more
ports then you can add more mangle rules and mark the traffic as Cable,
all the traffic marked as Cable will use the Cable routing table so they
will go to Cable modem.