But the name ???foo.com??? is
canonical and valid.
??? The name of the sending client. That is, it must not be any of the names by
which the server knows itself, such as ???localhost??? or ???127.0.0.1.???
Although RFC2821 requires that these characteristics be met by the client, it also
prohibits rejection based on bad values. If your site is besieged by spam, however,
the niceties of RFC2821 may not seem worth following. If you desire to reject badly
formed HELO/EHLO hostnames, you may do so by using this FEATURE(block_bad_helo):
FEATURE(`block_bad_helo??)
Note that, with this feature defined, certain clients are not checked at all:
??? If the client has authenticated with AUTH, the HELO/EHLO host is not checked.
??? If the client is listed with the RELAY_DOMAIN mc macro (?§7.4.1.1 on page 269) or in
the file specified by the RELAY_DOMAIN_FILE mc macro (?§7.4.1.2 on page 269), that
client??™s HELO/EHLO hostname is not checked.
Otherwise, all other clients have the host part of the HELO/EHLO greeting checked:
??? If the hostname exists as part of the server??™s $=w class (?§22.6.16 on page 876), the
HELO/EHLO command is rejected.
??? If the hostname exists as an IP address in the server??™s $=w class (?§22.6.16 on page
876), the HELO/EHLO command is rejected.
??? If the hostname has only an initial dot, a final dot, or no dot at all, the HELO/EHLO
command is rejected.
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