This is not-quite SMTPUTF8 compliant; if we have 8bit data and
the remote host advertises the relevant capabilities, we'll
add the appropriate parameters to MAIL FROM.
It's non-compliant in that we're supposed to raise an error
if we have 8bit and the destination doesn't support it.
Here we're assuming that the remote host will raise an error
if it won't accept it.
refs: https://github.com/KumoCorp/kumomta/issues/327
Wez Furlong
45802ff8a6