Thanks for the replies and suggestions.

The info in the bounceback message provided me with a Symantic website URL and error 553 as a reference. That error reference on the Symantic site indicated that my public IP was listed in the Spamhaus PBL. Primary fix is removal from the PBL. Secondary fix is to have my email address added to the Symantic Cloud Approved Sender List at the recipients organisation, so will be trying to do that tomorrow.

But contrary to the primary fix above, the FAQ's on the Spamhaus website states the following:

 

Help! My IP address is on the PBL! What should I do?

Nothing, in most cases. Read through this FAQ for further explanations.

The first thing to know is: THE PBL IS NOT A BLACKLIST. You are not listed for spamming or for anything you have done. The PBL is simply a list of all of the world's dynamic IP space, i.e: IP ranges normally assigned to ISP broadband customers (DSL, DHCP, PPP, cable, dialup). It is perfectly normal for dynamic IP addresses to be listed on the PBL. In fact all dynamic IP addresses in the world should be on the PBL. Even static IPs which do not send mail should be listed in the PBL.

PBL listings do not prevent you sending email unless your email program is not authenticating properly when it connects to your ISP or to your company's mail server. This can happen if you have forgotten to turn on 'SMTP Authentication' or if you have switched 'SMTP Authentication' off by mistake.

 

As per my original post, I have SMTP Authentication selected and am using Port 587.