Don't allow blind aliases with sendmail/pipe mts. Before this change,

they would be fed to sendmail, which would reject them anyway.  This
change catches that in post and presents a more helpful error message
to the user (at the "What now?" prompt, if used).

A possible future enhancement might be, with sendmail/pipe, to expand
out blind aliases and put the addresses in Bcc: header fields, but
then the recipients would get messages with the Blind-Carbon-Copy
indication.  (Note that Dcc isn't supported with sendmail/pipe,
either.)

Just for information:  old spost fed the addresses of blind aliases to
sendmail, making them visible to all recipients.  Oops.