501 Connection rejected by policy [7.7]

Hi there, I run my own mail server for my consulting company, hosted at Digital Ocean in New York. It has had the same IP address for a few years, I publish an SPF record, it has PTR records for both IPv6 and IPv4, and it does not send spam or act as an open relay or anything like that. However I still get the error 501 Connection rejected by policy [7.7] 9612 whenever I send to a company that uses Symantec Email Security (Messagelabs). Can you please delist my IP address, which is 192.241.243.54, and the domain is noware.co.uk.

thanks,

Neil Cook

dig mail.noware.co.uk

; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.30.rc1.el6 <<>> mail.noware.co.uk

;; global options: +cmd

;; Got answer:

;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 48300

;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:

;mail.noware.co.uk.        IN    A

;; ANSWER SECTION:

mail.noware.co.uk.    21599    IN    A    192.241.243.54

dig -x 192.241.243.54

; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.30.rc1.el6 <<>> -x 192.241.243.54

;; global options: +cmd

;; Got answer:

;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 40855

;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:

;54.243.241.192.in-addr.arpa.    IN    PTR

;; ANSWER SECTION:

54.243.241.192.in-addr.arpa. 1799 IN    PTR    mail.noware.co.uk.