The problem is that you don't sent to the destination SMTP server. You send to your SMTP server. That goes at least one hop via SMTP and eventually ends up on the destination's domain server.
So even if I setup and host my own SMTP server, and even if I verify the TLS certs on my side, I have no way to verify that I'll get (1) A TLS connection (2) with an authenticated cert all the way to the ultimate destination.
It's beyond my control to ensure that I'm secured when emailing to an arbitrary domain with arbitrary configuration.
It's quite likely however that Lavabit, being a service that focusses on privacy, delivers enough emails directly to the target server over a secure protocol to cause problems for the NSA in this investigation.
So even if I setup and host my own SMTP server, and even if I verify the TLS certs on my side, I have no way to verify that I'll get (1) A TLS connection (2) with an authenticated cert all the way to the ultimate destination.
It's beyond my control to ensure that I'm secured when emailing to an arbitrary domain with arbitrary configuration.