IronPort SMTP Routes Overview

IronPort 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 to This
mapping causes any email with in the Envelope Recipient address to go instead to The system performs an “MX” lookup on, 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 Cisco IronPort AsyncOS operating system allows up to forty thousand
(40,000) SMTP Route mappings to be configured for your Cisco IronPort appliance.

This feature also allows host “globbing.” If you specify a partial domain, such as, then
any domain ending in matches the entry. For instance, and 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

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 and in the SMTP Routes table, the entry for will be used because it is the
more specific entry — even if it appears after the less specific 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 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.



SMTP Routes Limits

You can define up to 40,000 routes. The final default route of ALL is counted as a route against this limit.

Therefore, you can define up to 39,999 custom routes and one route that uses the special keyword ALL.


Managing SMTP Routes via the GUI

Use the Network > SMTP Routes page to manage SMTP Routes on your Cisco IronPort appliance. You

can add, modify, and delete mappings in the table. You can export or import the SMTP Routes entries.

Figure 2-1 SMTP Routes Page


Adding SMTP Routes

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