I find that emails to some BT addresses are bouncing. Several days after send them I get this automated response
Hi. This is the qmail-send program at bounce.mail.unifiedlayer.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>:
Connected to 65.20.0.49 but sender was rejected.
Remote host said: 421 Too many messages (1.5.6.1)
I'm not going to try again; this message has been in the queue too long.
--- Enclosed are the original headers of the message.
David