Thanks for your anwser,
This is a good solution for external service. But i have a question for internal service.
If i use two email servers and i configure this with DNS high priority one of them, in case that my first server fail and the second get emails, everything was ok. But when the first is up again then i can think that i lost emails because when my user final check its email, Outlook always check the email server that have higher priority then Outlook never download the emails that arrived to the second server.
There's a way to the SME second server forward to the first server SME Server the emails that it received when the firts server was down??.
What about ETRN o "multi-drop" option?
Thanks,
Miguel!