Bug#849770: exim4: bogus reject response on overlong lines

[Rúben Leote Mendes]

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