The format of the header lines that
sendmail(1M)
inserts into the message are defined by the H line.
The syntax of this line is:
H [?mflags?] hname:htemplate
Continuation lines in this specification are reflected directly into the outgoing message. The htemplate is macro-expanded before insertion into the message. If the mflags (surrounded by question marks) are specified, at least one of the specified flags must be stated in the mailer definition for this header to be automatically output. If one of these headers is in the input it is reflected to the output regardless of these flags.