:: SEND ACK ::
If a mail contains "/ACK" or "/ack" an acknowledge is sent from target BBS
to sender call to tell that the mail has received its target. Such acknowledge
mails have "ACK:..." in the title and don't contain any R:-header lines.
An acknowledge is sent after first read. Only if the mail is read with
option "-G", no acknowledge is sent. /ACK is not working with BIN or 7+ mails.