Symptoms
Client have two servers in their farm.
The new server is correctly sending emails out via the exchange 2010, the original server is sending some emails via SMTP even thought it is configured for Exchange and SmartActions.
Diagnoses
After investigation we noticed that the node one was pointing to the incorrect SMTP server.
Resolution
We edited the connection strings so it points to the correct host.
We also added the SMTP connection string to the new node