[exim] 421 Unexpected failure, please try later

Folks

Some of my users have been getting 421 errors when trying to send email
from their systems via my servers. None of this is logged by Exim on my
server. The customers logs are getting something like

SMTP command failed when talking to smtp.xxxxxx.net: <<< RCPT
TO:<xxxxx@???> >>> 421 Unexpected failure, please try later



I'm not sure what the 421 means. I had though that it meant that there
were no available connections left but it appears to be set for other
reasons too (like AOL use it when rejecting mail because the sending IP
has been flagged for spamming).



What does it mean?

Can I log whenever my server sends this message to connecting hosts?

How do I measure what the current connections are for my server?



Thanks

Martin