Fri Oct 30 21:08:16 server dovecot[27407]: lda(pid 27407 user userW): Panic: file istream.c: line 104 (i_stream_read): assertion failed: ((size_t)ret+old_size == _stream->pos - _stream->skip)i.e. was the input stream seekable?
Postfix's pipe process invoked dovecot-lda so no, the input stream was not seekable.
I'll try your patches. Thanks.
