I have had a lot of this nonsense with BT customers and nobody else, with the same jargon-rich notification from BT as quoted above and no reply when i have emailed BT asking for explanation and/or advice after no help when phonong. It seems to happen with all three of my isps but I haven't checked all modes yet. Even subdividing the members-list down to 1 addressee with 2 ccs gets bounced. It definitely isn't limited to sending via relay.plus.net AND nobody without BT in their name is doing it.
It has actually prevented me from communicating with a third of our membership other than sending an individual message to each - no message with either cc or bcc gets through - NOT EVEN to the primary addressee (that ought to be a punishable offence). I am so angry that I would pay for a copy of a respected computer magazine in order to publicise this disgraceful treatment in the next issue.
So far I have been advised to set up a group of members as one addressee and I just noticed my us-based host has a format for doing that.
Before I try that, here are other points arising from the foregoing replies (1) - where did this limit of 50 addressees come from, who said and who agreed to that - what use is CC if it's arbitrarily limited? (2) - Not every community member knows a lot and it would be a good idea for contributors to define every abbreviation she/he uses. ISP = internet service provider (3) what is SPF?