Cisco Cisco Email Security Appliance C160 Guía Del Usuario
24-2
Cisco AsyncOS 9.0 for Email User Guide
Chapter 24 Configuring Routing and Delivery Features
Routing Email for Local Domains
•
•
•
•
•
•
SMTP Routes Overview
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
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 AsyncOS operating system allows up to forty thousand (40,000) SMTP
Route mappings to be configured for your appliance. (See
email is being redirected. The AsyncOS operating system allows up to forty thousand (40,000) SMTP
Route mappings to be configured for your appliance. (See
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,
fred@foo.example.com
and
wilma@bar.example.com
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
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
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.
Default SMTP Route
You can also define a default SMTP route with the special keyword
ALL
. If a domain does not match a
previous mapping in the SMTP Routes list, it defaults to being redirected to the MX host specified by
the
the
ALL
entry.
When you print the SMTP Routes entries, the default SMTP route is listed as
ALL:
. You cannot delete
the default SMTP route; you may only clear any values entered for it.
Configure the default SMTP route via the Network > SMTP Routes page or the
smtproutes
command.