You put me in the right direction.

I've tried changing the IP address in the /etc/hosts:

- When I try the second SMT server for Frankfurt region ( --> still does not work

- When I try with one of the SMT servers from Ireland region ( --> it works

So it seems to be a problem with both Frankfurt SMT servers.
It still a problem for us as we want to deploy multiple instances automatically with CloudFormation in the Frankfurt region, and the instance came initialized with the Frankfurt SMT.