This chapter explains
the features that affect routing and delivery of email traveling through the
Cisco Content Security Management appliance, and use of the SMTP Routes page
SMTP Routes allow you
to redirect all email for a particular domain to a different mail exchange (MX)
host. For example, you could make a mapping from example.com to
groupware.example.com . This mapping causes any email with @example.com in the
Envelope Recipient address to go instead to groupware.example.com . The system
performs an “MX” lookup on groupware.example.com , and then performs an “A”
lookup on the host, just like a normal email delivery. This alternate MX host
does not need to be listed in DNS MX records and it does not even need to be a
member of the domain whose email is being redirected. The operating system
allows up to ten thousand (10,000) SMTP Route mappings to be configured for
your Cisco Content Security appliance. (See
SMTP Routes Limits.)
This feature also
allows host “globbing.” If you specify a partial domain, such as example.com ,
then any domain ending in example.com matches the entry. For instance,
email@example.com and firstname.lastname@example.org both match the mapping.
If a host is not
found in the SMTP Routes table, an MX lookup is performed using DNS. The result
is not re-checked against the SMTP Routes table. If the DNS MX entry for
foo.domain is bar.domain , any email sent to foo.domain is delivered to the
host bar.domain . If you create a mapping for bar.domain to some other host,
email addressed to foo.domain is not affected.
In other words,
recursive entries are not followed. If there is an entry for a.domain to
redirect to b.domain , and a subsequent entry to redirect email for b.domain to
a.domain , a mail loop will
not be created.
In this case, email addressed to a.domain will be delivered to the MX host
specified by b.domain , and conversely email addressed to b.domain will be
delivered to the MX host specified by a.domain .
The SMTP Routes table
is read from the top down for every email delivery. The most specific entry
that matches a mapping wins. For example, if there are mappings for both
host1.example.com and example.com in the SMTP Routes table, the entry for
host1.example.com will be used because it is the more specific entry — even if
it appears after the less specific example.com entry. Otherwise, the system
performs a regular MX lookup on the domain of the Envelope Recipient.