UCEProtect is a DNS blocklist service consisting of three block lists that block either a single IP (dnsbl-1 Level 1), a subnet (dnsbl-2 Level 2), or an ASN (dnsbl-3 Level 3).
Barracuda has observed that some organizations are using UCEProtect Level 3 as a blocklist on their mail systems. When UCEProtect blocks AWS ASN AS16509 and your recipient organization uses UCEProtect Level 3 as a blocklist, they are unable to receive mail from you when you send outbound mail using the Barracuda Email Security Service. This can happen even though none of the Barracuda Email Security Service IPs are specifically blocked by UCEProtect. Unfortunately, this is out of Barracuda's control. Barracuda recommends the recipient organization to adjust their mail systems to follow the recommended best practices when using UCEProtect Level 3 blocklist.
Based on UCEProtect's recommendations, the use of Level 3 is to incorporate it into a scoring system, to give e.g. 2 points on a ‘match’ where 5 or more points trigger a spam tag.
For more information, see:
- AWS Block List FAQ: Can Amazon SES have its IP addresses removed from a block list?
- Microsoft: Recipient environment that uses UCEProtect for mail security
Recipient organizations should also allow the following IP address ranges in order to receive mail from the Barracuda Email Security Service: