Here, on my box are the files that can be used to rpm install ssmtp on rhel7 or centos7. Based on el6 rpm from epel and fixes this linking problem:
/bin/ld: note: 'X509_free@@libcrypto.so.10' is defined in DSO /lib64/libcrypto.so.10 so try adding it to the linker command line
hope it helps.