Muttrc not source when reading from stdin

[Roger Cornelius <rac@xxxxxxxxxxx>]

mutt 1.5.16, 1.5.17
SCO OSR507 & OSR6

I have "hostname=<somehost>" in the system Muttrc file.  If I invoke
mutt interactively to send a message, e.g. "mutt someuser", the hostname
setting is honored and the From and To headers both contain <somehost> as
expected.  But if mutt reads it's input from stdin,
e.g. "echo test | mutt -s test someuser", the hostname= setting is
ignored and neither the From nor To headers reflect the hostname
setting.  My .signature is also not appended to the message.

Is this intended behavour, or am I missing something (or a bug)?
-- 
Roger Cornelius        rac@xxxxxxxxxxx