We received an NDR recently from barracuda networks spam filter with a 550 error code. It also came back with the SMTP address we were trying to send to as well as :blocked at the end. E.g.

Final-Recipient: rfc822; your@email.com
Action: failed
Status: 5.0.0
Remote-MTA: dns; d9590b.ess.barracudanetworks.com
Diagnostic-Code: smtp; 550 permanent failure for one or more recipients
    (your@email.com:blocked)

I tried all the basics making sure our IP had not been blacklisted : 
http://www.barracudacentral.org/reputation which it had not. I tried 
telneting to one of the SMTP servers to view the error, however this 
produced a malformed 550 error so not a good test.

Upon liaising with the postmaster at the receiving end, turns out the email had a domain name 
in the body which was on their Intent Database :
 https://techlib.barracuda.com/display/bessv10/intent+analysis+-+inbound+mail which is why it 
got blocked. Make sure you check your email for domain names if you get these.