More thoughts:
Hypothesis a): the headers "to:" and "from:" are not case sensitive, so 
"FROM:" and "From:" are equivalent.
Hypothesis b) therefore there should be only one of "to:" and "from:"

They they hold, the tests would need to be fixed first, then the question 
is: What is the right version of from: and to:? (Which one would the smtp 
protocol actually take?)

The next steps:
How can I verify the hypothesis?
idea i) look up SMTP standard
idea ii) write/use/adapt an example code and try it with a real smtp 
server/client