With the attached (the first test from T355-smime.sh) saved as foo.sh,
try running the following.

count=0; while ./foo.sh ; do
count=$((count + 1))
echo $count >> tests.log
done

After the somewhere between 1 and 200 iterations this hangs up for
me. Emacs is still running, the message-mode buffer has appropriate
looking stuff in it. There is nothing interesting in *Messages*, and the
buffer is still titled "*unsent mail*". When sending manually, I get
a message that there is no key for test_suite@notmuchmail.org

After digging through mml-secure for a while, I realized that the
problem is not actually in emacs, but in our test harness for gpgsm. I
added the gpgsm -K to the script, and you can see that the
test_suite@notmuchmail.org key is not being added.

I think that means there is some kind of intermittent failure with the
following pipeline
    
    openssl pkcs12 -export -passout pass: -inkey "$NOTMUCH_SRCDIR/test/smime/key+cert.pem" \
	< "$NOTMUCH_SRCDIR/test/smime/test.crt" | \
	gpgsm --batch --no-tty --no-common-certs-import --pinentry-mode=loopback --passphrase-fd 3 \
	      --disable-dirmngr --import  >"$GNUPGHOME"/import.log 2>&1 3<<<''

I've attached "import.log". I _think_ the failure reported is the
notmuchmail.org key, and the success is the 'Bob Babbage' key,