It seems you are missing the size check in the DATA ACL and so the
messages are refused in the transport that, as you report, gives a very
unhelpful error.
The "acl/40_exim4-config_check_data" that ships with stretch should
refuse the messages with "maximum allowed line length is 998 octets, got
$max_received_linelength", which is much more helpful:
acl_check_data:
# Deny if the message contains an overlong line. Per the standards
# we should never receive one such via SMTP.
#
.ifndef IGNORE_SMTP_LINE_LENGTH_LIMIT
deny message = maximum allowed line length is 998 octets, \
got $max_received_linelength
condition = ${if > {$max_received_linelength}{998}}
.endif
I had to disable the check because it was blocking many legitimate
messages, so it should probably not be the default.
Rúben
