nits with marid-core-02

Two small things

1.  In 7.2/7.3 and 7.4 why is the use of Resent-From or Sender a SHOULD 
rather than a MUST?  Is there an alternative that would be acceptable and 
what would be gained by that?

2.  Typo in 7.4 first sentence.  Says 'than' where it should say 'that'.

Jay Daley