Founded in 2003, Barracuda Networks provides anti-spam and security-related hardware and services and was believed to have more than 150,000 clients as of 2014.
Looks like if Barracuda hosts your spam filtering or mail services, you might not be receiving email right now. Multiple folks are telling me that they're having trouble connecting to Barracuda servers to deliver mail. The Register (UK) has mention of Barracuda downtime today as well.
As of 2:26 pm Eastern Time on Wednesday, November 2, 2016, Barracuda's status website says: "Investigating - Customers are experiencing delays with inbound message delivery. Outbound is unaffected. [...] Engineering and Operations teams are still working to resolve delays in mail delivery."
Update: November 3, 2016: "Barracuda Networks is still continuing to see a large number of inbound connections from unverified sources for customers using Essentials for Email Security and Cloud Protection Layer. We have successfully filtered and are actively monitoring the situation while taking the appropriate actions when needed. Email processing has returned to normal. Previously delayed emails are now being accepted and processed."