All strings used in mail messages are assumed to conform to their corresponding SMTP specifications, except as noted otherwise.
Note that passing already-quoted strings would be fine, since then there are no non-ASCII characters.
The return value is an output port into which the client must write the message. Clients are urged to use close-output-port on the return value as soon as the necessary text has been written, so that the sendmail process can complete.
The from argument can be any value; of course, spoofing should be used with care. If it is #f, no “From:” header is generated, which usually means that your sendmail program will fill in the right value based on the user.
Lines that contain a single period do not need to be quoted.
sendmail@ : unit?
sendmail^ : signature
Includes everything exported by the net/sendmail module.