Message contains NUL characters

[Ross Boylan]

I've been getting 
LMTP error after end of data: 554 5.6.0 Message contains NUL characters
from cyrus-imapd-2.2 2.2.13-14+lenny4 running on Lenny.  Google and
inspecting the original message makes me suspect the problem is long
lines in the message, rather than embedded NUL's.  The messages are not
spam, though I know such lines are non-compiant.

There were also messages suggesting the long lines problem had been
fixed.  However, I don't see anything in the Debian changelogs or
upstream's (looking at 2.4) about a this issue.

Any suggestions about the best way to handle this?  There are some
obstacles to upgrading to squeeze, which is still 2.2 anyway.

exim is my MTA.

Thanks.
Ross Boylan