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Configuring DNS Routing for Improved Performance

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If you want or need to filter your outbound mail through the Barracuda Email Security Service (BESS) we recommend setting up your outbound mail server to use the outbound smarthost that is shown on your Barracuda Email Security Service Domains Settings page.

There are two problems that can occur when pointing all your outbound mail to our service:

  • If you send to a large group of users it may take longer for us to verify all the users then your mail servers timeout (default timeout is 5 minutes) this will result in a sender timeout and the mail will not be delivered. One way to resolve this is to increase your outbound connector timeout.
  • Some mail servers (Exchange in particular), when sending mass mailings (one email to multiple addresses in multiple domains), will see a failure to one address as a failure to all addresses. Exchange incorrectly reports the failure to one recipient as a failure of the entire message.

Full Instructions

Refer to Setting Up DNS Routing in Exchange 2013 or later for detailed instructions for this configuration.

DNS Routing

You can force your mail server to break this outbound mail into per domain packets which will limit the delays/failures in mail delivery. This is called DNS Routing.

You can configure DNS routing in either of these two methods:

  • DNS server on your network – Deliver mail directly to the domain,s mail server
  • Barracuda DNS server – Deliver mail for all domains to Barracuda Email Security Service.

When using DNS routing, your mail server will break the mass mailing into per domain groups but still deliver the mail to BESS. The BESS DNS routing servers are configured to return the Barracuda Inbound hostname for your region as the MX record for all domains.

Configuring DNS Routing
Local DNS Routing Server

Configure your local DNS Routing Server to return the same hostname (see below) as the MX record for all domains.

BESS DNS Routing Server

Configure your mail server to use the BESS DNS Server based on your location (region).

Note: This is for your DNS routing ONLY and not for other DNS requests.

IP Addresses

Enter both IP addresses into your DNS configuration to provide better redundancy.

  • AU (Australia) IP Addresses – and
  • CA (Canada) IP Addresses – and
  • DE (Germany) IP Addresses and
  • UK IP Addresses – and
  • US (United States) IP Addresses – and
DNS Server for MX Lookups

This DNS server is for MX lookups only and returns for all MX queries.

This allows your mail server to break up outbound mail into "per domain" packets, but still send all the mail through the ESS service.

  • AU –
  • CA –
  • DE –
  • UK
  • US –

Important to Note

Consider the following points before configuring DNS routing.

Loss of Redundancy

If you use DNS routing for your outbound mail you will lose the redundancy the normal smarthost provides. Barracuda does not foresee any outage with these servers, but it is something anyone using DNS routing must consider.

Exchange Users

Another solution would be to NOT filter your mail through the Barracuda Email Security Service when your mail server is Exchange. This will allow you to deliver mail normally using DNS routing directly to the destination mail servers.

Too Many Recipients

If you are only seeing connection timeouts due to too many recipients, then you must dramatically increase the SMTP timeout of the sending server. The default is 5 minutes, which is often just not long enough when using the Barracuda Email Security Service as your relay.

Additional Information

Microsoft documents that might be of assistance with this topic:


This article was originally published as Solution #00006828 

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