Subscribing to this forum didn't work with default ispcp configuration

[rbtux]

Well take the appropriate RFC: 2821.

"These commands are used to identify the SMTP client to the SMTP
server. The argument field contains the fully-qualified domain name
of the SMTP client if one is available. In situations in which the
SMTP client system does not have a meaningful domain name (e.g., when
its address is dynamically allocated and no reverse mapping record is
available), the client SHOULD send an address literal (see section
4.1.3), optionally followed by information that will help to identify
the client system."

Well I think this is enough specific. But you always have to create a email policy for your or your customer needs. So I think the discussion is senseless because mail server configuration is a very individual thing...