Handle the HELO/EHLO command.
||xxfi_helo is called whenever the client sends a HELO/EHLO
command. It may therefore be called several times or even not at
all; some restrictions can be imposed by the MTA
||Do nothing; return SMFIS_CONTINUE.
||Opaque context structure.
||Value passed to HELO/EHLO command, which should be the domain
name of the sending host (but is, in practice, anything the sending
host wants to send).