Yup, always "\r\n" (actually I got a little paranoid and always make a habit to write "\015\012", because sometimes I tinker on Windows).

Timing out: don't think so. The timestamps on my log clearly show consistently only 1-2 second between done receiving DATA and sending the 250 line.

Anyway, I'm gonna try to have a session with my client and will do tcpdump and strace as well. Thanks. (Argh, it's so frustrating not being able to reproduce this myself...)