How does mail flow through the Barracuda Email Security Service from sender to recipient
The first thing that is important to know is that the Barracuda Email Security Service (ESS) is not a normal mail service. It does not accept incoming mail, close the connection, process the mail and then deliver it.
It is what is known as a relay or pass-through service. It does the following
1. The sender connects to the ESS service
2. They send the "mail from" and "rcpt to" commands
3. ESS reads the "rcpt to" and if
a.) If it is an ESS customer domain
b.) If it is outbound mail coming from an authorized customer IP
4. ESS connects to the domains destination server
5. ESS monitors the data between the sending and recipient servers
a.) it checks for intent (domains or addreses on our blocklists)
b.) viruses (signatures or malware)
c.) content filters - Barracuda, account or domain level
d.) sender filters - Barracuda, account, domain or user level
e.) recipient filters - Barracuda, account or domain level
6. If for any reason mail is found that should be blocked the connection to the destination is reset (dropped) and we return back to the sending server a 5xx blocked reason. The sending server is then responsible for generating an NDR back to the sender.
IMPORTANT NOTE: Mail that is deferred for any reason is NOT delivered by ESS.
This deferral is returned back to the sending server and it is responsible for retrying the mail. This retry will be a NEW line in the message log.
The best way to find out if a deferred piece of mail has been retried and delivered is to do a search of the message log for the sender and subject of the message.
Please reference our terms of service page in regards to bulk or mass mailing.