diff --git a/configure b/configure
index 0cfdaa6f..37368bda 100755
--- a/configure
+++ b/configure
@@ -494,7 +494,7 @@ int main () {
     if (error) return !! fprintf (stderr, "failed to instantiate parser with test/corpora/crypto/basic-encrypted.eml\n");
 
     body = GMIME_MULTIPART_ENCRYPTED(g_mime_message_get_mime_part (g_mime_parser_construct_message (parser, NULL)));
-    if (body == NULL) return !!	fprintf (stderr, "did not find a multipart encrypted message\n");
+    if (body == NULL) return !! fprintf (stderr, "did not find a multipart encrypted message\n");
 
     output = g_mime_multipart_encrypted_decrypt (body, GMIME_DECRYPT_EXPORT_SESSION_KEY, NULL, &decrypt_result, &error);
     if (error || output == NULL) return !! fprintf (stderr, "decryption failed\n");
@@ -536,6 +536,82 @@ EOF
     if [ -n "$TEMP_GPG" -a -d "$TEMP_GPG" ]; then
         rm -rf "$TEMP_GPG"
     fi
+
+    # see https://github.com/jstedfast/gmime/pull/90
+    # should be fixed in GMime in 3.2.7, but some distros might patch
+    printf "Checking for GMime X.509 certificate validity... "
+
+    cat > _check_x509_validity.c <<EOF
+#include <stdio.h>
+#include <gmime/gmime.h>
+
+int main () {
+    GError *error = NULL;
+    GMimeParser *parser = NULL;
+    GMimeApplicationPkcs7Mime *body = NULL;
+    GMimeSignatureList *sig_list = NULL;
+    GMimeSignature *sig = NULL;
+    GMimeCertificate *cert = NULL;
+    GMimeObject *output = NULL;
+    GMimeValidity validity = GMIME_VALIDITY_UNKNOWN;
+    int len;
+
+    g_mime_init ();
+    parser = g_mime_parser_new ();
+    g_mime_parser_init_with_stream (parser, g_mime_stream_file_open("$srcdir/test/corpora/pkcs7/smime-onepart-signed.eml", "r", &error));
+    if (error) return !! fprintf (stderr, "failed to instantiate parser with test/corpora/pkcs7/smime-onepart-signed.eml\n");
+
+    body = GMIME_APPLICATION_PKCS7_MIME(g_mime_message_get_mime_part (g_mime_parser_construct_message (parser, NULL)));
+    if (body == NULL) return !!	fprintf (stderr, "did not find a application/pkcs7 message\n");
+
+    sig_list = g_mime_application_pkcs7_mime_verify (body, GMIME_VERIFY_NONE, &output, &error);
+    if (error || output == NULL) return !! fprintf (stderr, "verify failed\n");
+
+    if (sig_list == NULL) return !! fprintf (stderr, "no GMimeSignatureList found\n");
+    len = g_mime_signature_list_length (sig_list);
+    if (len != 1) return !! fprintf (stderr, "expected 1 signature, got %d\n", len);
+    sig = g_mime_signature_list_get_signature (sig_list, 0);
+    if (sig == NULL) return !! fprintf (stderr, "no GMimeSignature found at position 0\n");
+    cert = g_mime_signature_get_certificate (sig);
+    if (cert == NULL) return !! fprintf (stderr, "no GMimeCertificate found\n");
+    validity = g_mime_certificate_get_id_validity (cert);
+    if (validity != GMIME_VALIDITY_FULL) return !! fprintf (stderr, "Got validity %d, expected %d\n", validity, GMIME_VALIDITY_FULL);
+
+    return 0;
+}
+EOF
+    if ! TEMP_GPG=$(mktemp -d "${TMPDIR:-/tmp}/notmuch.XXXXXX"); then
+        printf 'No.\nCould not make tempdir for testing X.509 certificate validity support.\n'
+        errors=$((errors + 1))
+    elif ${CC} ${CFLAGS} ${gmime_cflags} _check_x509_validity.c ${gmime_ldflags} -o _check_x509_validity \
+            && echo disable-crl-checks > "$TEMP_GPG/gpgsm.conf" \
+            && echo "4D:E0:FF:63:C0:E9:EC:01:29:11:C8:7A:EE:DA:3A:9A:7F:6E:C1:0D S" >> "$TEMP_GPG/trustlist.txt" \
+            && GNUPGHOME=${TEMP_GPG} gpgsm --batch --quiet --import < "$srcdir"/test/smime/ca.crt
+    then
+        if GNUPGHOME=${TEMP_GPG} ./_check_x509_validity; then
+            gmime_x509_cert_validity=1
+            printf "Yes.\n"
+        else
+            gmime_x509_cert_validity=0
+            printf "No.\n"
+            if pkg-config --exists "gmime-3.0 >= 3.2.7"; then
+                cat <<EOF
+*** Error: GMime fails to calculate X.509 certificate validity, and
+is later than 3.2.7, which should have fixed this issue.
+
+Please follow up on https://github.com/jstedfast/gmime/pull/90 with
+more details.
+EOF
+                errors=$((errors + 1))
+            fi
+        fi
+    else
+        printf 'No.\nFailed to set up gpgsm for testing X.509 certificate validity support.\n'
+        errors=$((errors + 1))
+    fi
+    if [ -n "$TEMP_GPG" -a -d "$TEMP_GPG" ]; then
+        rm -rf "$TEMP_GPG"
+    fi
 else
     have_gmime=0
     printf "No.\n"
@@ -1043,7 +1119,7 @@ for flag in -Wmissing-declarations; do
 done
 printf "\n\t%s\n" "${WARN_CFLAGS}"
 
-rm -f minimal minimal.c _libversion.c _libversion _libversion.sh _check_session_keys.c _check_session_keys
+rm -f minimal minimal.c _libversion.c _libversion _libversion.sh _check_session_keys.c _check_session_keys _check_x509_validity.c _check_x509_validity
 
 # construct the Makefile.config
 cat > Makefile.config <<EOF
@@ -1334,6 +1410,9 @@ NOTMUCH_HAVE_XAPIAN_DB_RETRY_LOCK=${WITH_RETRY_LOCK}
 # Which backend will Xapian use by default?
 NOTMUCH_DEFAULT_XAPIAN_BACKEND=${default_xapian_backend}
 
+# Whether GMime can verify X.509 certificate validity
+NOTMUCH_GMIME_X509_CERT_VALIDITY=${gmime_x509_cert_validity}
+
 # do we have man pages?
 NOTMUCH_HAVE_MAN=$((have_sphinx))
 
diff --git a/test/T355-smime.sh b/test/T355-smime.sh
index 117fa2b9..f8e8e396 100755
--- a/test/T355-smime.sh
+++ b/test/T355-smime.sh
@@ -187,13 +187,16 @@ test_valid_json "$output"
 test_begin_subtest "Verify signature on PKCS#7 SignedData message"
 test_subtest_known_broken
 output=$(notmuch show --format=json id:smime-onepart-signed@protected-headers.example)
+
 test_json_nodes <<<"$output" \
-                'crypto:[0][0][0]["crypto"]["signed"]["status"][0]={
-                        "created" : 1574813489,
-                        "expires" : 2611032858,
-                        "fingerprint" : "702BA4B157F1E2B7D16B0C6A5FFC8A7DE2057DEB",
-                        "userid" : "CN=Alice Lovelace",
-                        "status" : "good"
-                     }'
+                'created:[0][0][0]["crypto"]["signed"]["status"][0]["created"]=1574813489' \
+                'expires:[0][0][0]["crypto"]["signed"]["status"][0]["expires"]=2611032858' \
+                'fingerprint:[0][0][0]["crypto"]["signed"]["status"][0]["fingerprint"]="702BA4B157F1E2B7D16B0C6A5FFC8A7DE2057DEB"' \
+                'status:[0][0][0]["crypto"]["signed"]["status"][0]["status"]="good"'
+
+test_begin_subtest "Verify signature on PKCS#7 SignedData message signer User ID"
+test_subtest_known_broken
+test_json_nodes <<<"$output" \
+                'userid:[0][0][0]["crypto"]["signed"]["status"][0]["userid"]="CN=Alice Lovelace"'
 
 test_done
diff --git a/test/T356-protected-headers.sh b/test/T356-protected-headers.sh
index 520cb71c..5fd27434 100755
--- a/test/T356-protected-headers.sh
+++ b/test/T356-protected-headers.sh
@@ -163,8 +163,13 @@ for variant in multipart-signed onepart-signed; do
                     'signed_subject:[0][0][0]["crypto"]["signed"]["headers"]=["Subject"]' \
                     'sig_good:[0][0][0]["crypto"]["signed"]["status"][0]["status"]="good"' \
                     'sig_fpr:[0][0][0]["crypto"]["signed"]["status"][0]["fingerprint"]="702BA4B157F1E2B7D16B0C6A5FFC8A7DE2057DEB"' \
-                    'sig_uid:[0][0][0]["crypto"]["signed"]["status"][0]["userid"]="CN=Alice Lovelace"' \
                     'not_encrypted:[0][0][0]["crypto"]!"decrypted"'
+    test_begin_subtest "verify signed PKCS#7 subject ($variant) signer User ID"
+    if [ $NOTMUCH_GMIME_X509_CERT_VALIDITY -ne 1 ] || [ "$variant" != multipart-signed ]; then
+        test_subtest_known_broken
+    fi
+    test_json_nodes <<<"$output" \
+                    'sig_uid:[0][0][0]["crypto"]["signed"]["status"][0]["userid"]="CN=Alice Lovelace"'
 done
 
 for variant in sign+enc sign+enc+legacy-disp; do
@@ -175,8 +180,12 @@ for variant in sign+enc sign+enc+legacy-disp; do
                     'signed_subject:[0][0][0]["crypto"]["signed"]["headers"]=["Subject"]' \
                     'sig_good:[0][0][0]["crypto"]["signed"]["status"][0]["status"]="good"' \
                     'sig_fpr:[0][0][0]["crypto"]["signed"]["status"][0]["fingerprint"]="702BA4B157F1E2B7D16B0C6A5FFC8A7DE2057DEB"' \
-                    'sig_uid:[0][0][0]["crypto"]["signed"]["status"][0]["userid"]="CN=Alice Lovelace"' \
                     'encrypted:[0][0][0]["crypto"]["decrypted"]={"status":"full","header-mask":{"Subject":"..."}}'
+    test_begin_subtest "confirm signed and encrypted PKCS#7 subject ($variant) signer User ID"
+    test_subtest_known_broken
+    test_json_nodes <<<"$output" \
+                    'sig_uid:[0][0][0]["crypto"]["signed"]["status"][0]["userid"]="CN=Alice Lovelace"'
+
 done
 
 test_begin_subtest "confirm encryption-protected PKCS#7 subject (enc+legacy-disp)"