To reproduce, disable S/MIME support and send a message without signing it.

11:46:44/4480/message.cpp:get_recipients: got 1 recipients:
11:46:44/4480/message.cpp:get_recipients: `aheinecke@intevation.de'
11:46:44/4480/mapihelp.cpp:mapi_create_attach_table: message has 0 attachments
11:46:44/4480/Writing part of length 7 (body) filename=`[none]'
11:46:44/4480/ writing ascii of length 7
11:46:44/7524/ERROR/engine-assuan.c:worker_start_write: [ input:00003338] write
error: The pipe is being closed. (232)
11:46:44/4480/engine.c:engine_wait: filter 16389F90 ready: Internal error
11:46:44/4480/engine.c:engine_cancel: filter 16389F90: canceled