Cannot send via relay.plus.net to specific gmail user

[#1]

Hi
As the title suggests I cannot send to a specific gmail user though all other users of gmail and other domains that I email via my home mail server are fine.  My mail server (Synology) reports a 552 error from relay.plus.net, which is a spam warning.  Would the spam filters be specific to a user rather than just a domain?  Is it relay.plus.net rejecting it or because gmail won't accept it?
Never had this problem before.
Any ideas?
Andy

[#2]

Hi Andy,
From the limited information you've provided, I'm guessing it's our outbound spam filtering that's trapping the message and refusing it at the SMTP layer. The rejection should be accompanied with a fingerprint string e.g. fT5S1p00B0RiwEC01T5T6U.
If you raise a support ticket containing that information then we should be able to find out what's happening and get it sorted. The reason it's happening is likely to be because the message contains a domain/email/website address that's on our anti-spam vendor's naughty list. It may even be the address you're trying to send to that's causing the problem.

[#3]

Hi Bob
Thanks for the info.  There was a string reference so your assumption is correct.
I've communicated gmail-to-gmail now but I'll raise a ticket to get to the bottom of the matter if only for future reference. 
Yours
Andy