As reported in Debian [BR], long line in message body causes error "530 5.2.0 Message is empty. Aborting.". This in turn can causes MTA (exim is cited in the bug report) to fail with 'Broken pipe' message for dspam router and use retries to send other emails to other recepients, so messages are deferred and delivered with delay.
The error stems from the fact that pop_buffer returns NULL when no newline is found (which is systematic for a line longer than 1023 bytes, the size of the buffer used in client_getline()). When this happens, the NULL is propagated from pop_buffer to client_getline, then to read_sock and finally process_connection returns the aformentioned error. Attach a patch to truncate line at 1023 bytes in pop_buffer by adding a newline just before the \0 when none is found. Note that I didn't try the patch so review are more than welcome.