Handle the HELO/EHLO command.
|Called When ||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 configuration.
||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).