dnsmasq (FTLDNS is based on dnsmasq 2.79) considers the domain smtp.strato.de as BOGUS, meaning invalid. There are some issues with dnsmasq, regarding DNSSEC. You can read more about it here.
For now, the best solution seems to be to either simply disable DNSSEC (on the pihole settings page) OR to implement the solution from DLER6, witch bassicly implies running pihole without DNSSEC and install unbound, witch will handle the DNSSEC evaluation far better.