mh-send-uses-spost-flag

mh-send-uses-spost-flag is a variable defined in `mh-comp.el'.
Its value is

nil

Documentation:
Non-nil means "send" uses "spost" to submit messages.

If the value of "postproc:" is "spost", you may need to set
this variable to t to tell MH-E to avoid using features of
"post" that are not supported by "spost". You'll know that
you'll need to do this if sending mail fails with an error of
"spost: -msgid unknown".