ACK messages
An ACK messages gives the confirmation that a PRIVATE arrived in tis destination back
to the sender.
How does an ACK message work ?
  - An ACK message is requested with the string 
 /ACK
 written by the sender inside his message in 1st column.
- When the message arrives in its destination, an ACK message is generated and sent back
    to the requester, including the date and eventually other informations.
Protocol
  - The SID of the BBS including this feature will have the letter "A"
- An ACK message is a short message including only text
- An ACK message is generated only on private mail, not on bulletins
- The message has a type "A" (private are "P", bulletins are
    "B")
- Only the first header forwarding line is generated, other BBS will not add their header
    forwarding line
- If the BBS which is forwarded does not include this feature, the message is changed to
    type "P" (Private).
- Type "A" message are not seen by users.
- When arriving back to its destination, the type of the ACK message is changed to type
    "P" (private) to be read by the requester.