Email 550 Error for non-existent address - sometimes

[#1]

I have 2 PN accounts. Both have Catch All switched to 'Off'
Account 1 does not return any error if sent an email to an address that has not been set up. (Though it used to about a year ago before the Cathch All option when I had the default 'blackholed')
Today I discover that Account 2 does return such errors.
Why the difference....? And does this explain the confusion from PN staff when I first reported the changed behaviour of Account 1 in a ticket.
(I never had account 2 'blackholed'. The default on Account 1 now picks up mails addressed to it... so I'm guessing 'blackhole' is now off following the setting of Catch All?)

[#2]

To my knowledge our mail system will work as follows...
Email sent to an invalid domain (ie. non existent username) either externally or from another PlusNet address will return a 5xx permanent rejection.
Email sent to a valid doman (read:username) but an invalid local part (bit before the '@' sign) will not be bounced back to the sender. This is to help prevent the problems that can be caused by backscatter
Where we have manually intervened (before the Manage My Mail tool), then there may be instances where certain local parts are blackholed and return a failed delivery notice.

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Today I discover that Account 2 does return such errors.

I can't really explain that as it doesn't fit with what I've said above. Is this a hosted domain or a .plus.com address? Where is the mail being sent from that's getting bounced? What outgoing mail server are you sending it through.