BT reversion to Yahoo…?

[#1]

On Friday I sent out a routine email via PN SMTP to a small distribution list – something I do regularly.
Yesterday I received a non-delivery bounce message for each such email destined for an @btinternet.com addressee; the others were apparently delivered successfully.
Oddly, the bounce message said
“Hi. This is the qmail-send program at yahoo.com.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.
<xxx@btinternet.com>:
Can't append to inbox for <xxx@btinternet.com>.  Temporary error
//I'm not going to try again; this message has been in the queue too long.
(with recipient names being replaced by xxx).
I cannot believe that BT has backed out a pile of the transfers earlier in the year from Yahoo to white-label email provider Critical Path. But equally the qmail bounce message appears to be a real yahoo one.
Before I re-send, any ideas pls? 

[#2]

Dealt with a similar complaint last week, I smell a BT problem.

[#3]

The MX record for btinternet.com points to servers on the domain cpcloud.co.uk (Presumably Critical Path)
Perhaps not all accounts are fully migrated yet?  May be the process stalled in light of this:
http://www.bbc.co.uk/news/technology-26480381