diff --git a/Documentation/SOGoInstallationGuide.asciidoc b/Documentation/SOGoInstallationGuide.asciidoc
index ac1a229bf..86822e2ee 100644
--- a/Documentation/SOGoInstallationGuide.asciidoc
+++ b/Documentation/SOGoInstallationGuide.asciidoc
@@ -2503,6 +2503,18 @@ Defaults to `NO` when unset.
 |Parameter used to enable S/MIME certificate management from the account editor of the
 preferences window.
 
+When a S/MIME signed message is displayed, the sender's certificate is
+saved in the matching contact of the user's address books (and replaces any
+existing one) only if the signature is valid and every signer certificate
+is issued for the sender's email address. A signature is valid only if the
+signer certificate chains to a certificate authority trusted by the system
+(OpenSSL default CA store, for example `/etc/ssl/certs`). Certificates that
+are self-signed or issued by a private CA are therefore not imported unless
+that CA is added to the system trust store (for example in
+`/usr/local/share/ca-certificates` followed by `update-ca-certificates` on
+Debian/Ubuntu). Note that some distributions only ship the CAs trusted for
+TLS servers, so the root CAs dedicated to S/MIME may also need to be added.
+
 Defaults to `YES` when unset.
 
 |U |SOGoSelectedAddressBook
diff --git a/UI/MailPartViewers/UIxMailPartSignedViewer.h b/UI/MailPartViewers/UIxMailPartSignedViewer.h
index b9f8e2585..0a759135e 100644
--- a/UI/MailPartViewers/UIxMailPartSignedViewer.h
+++ b/UI/MailPartViewers/UIxMailPartSignedViewer.h
@@ -30,11 +30,13 @@
 {
   BOOL processed;
   BOOL validSignature;
+  BOOL signersMatchSender;
   NSMutableArray *certificates;
   NSString *validationMessage;
 }
 
 - (BOOL) validSignature;
+- (BOOL) signersMatchSender;
 - (NSString *) validationMessage;
 - (NSArray *) smimeCertificates;
 - (NSDictionary *) certificateForSubject: (NSString *) subject
diff --git a/UI/MailPartViewers/UIxMailPartSignedViewer.m b/UI/MailPartViewers/UIxMailPartSignedViewer.m
index 397be6d5d..f45dd89c7 100644
--- a/UI/MailPartViewers/UIxMailPartSignedViewer.m
+++ b/UI/MailPartViewers/UIxMailPartSignedViewer.m
@@ -94,8 +94,8 @@
 - (void) _processMessage
 {
   NSData *signedData;
-  NSMutableArray *emails;
-  
+  NSMutableArray *emails, *signersEmails;
+
   STACK_OF(X509) *certs;
   X509_STORE *x509Store;
   BIO *msgBio, *inData;
@@ -117,6 +117,8 @@
   certs = NULL;
   certificates = [NSMutableArray array];
   emails = [NSMutableArray array];
+  signersEmails = [NSMutableArray array];
+  signersMatchSender = NO;
   validationMessage = nil;
 
   if (cms)
@@ -138,13 +140,21 @@
 	      char p[1024];
 	      int j;
 	      STACK_OF(OPENSSL_STRING) *emlst;
+	      NSMutableArray *signerEmails;
+	      NSString *email;
 
 	      x = sk_X509_value(certs, i);
 
+	      signerEmails = [NSMutableArray array];
 	      emlst = X509_get1_email(x);
 	      for (j = 0; j < sk_OPENSSL_STRING_num(emlst); j++)
-	          [emails addObject: [[NSString stringWithUTF8String: sk_OPENSSL_STRING_value(emlst, j)] lowercaseString]];
+		{
+		  email = [[NSString stringWithUTF8String: sk_OPENSSL_STRING_value(emlst, j)] lowercaseString];
+		  [emails addObject: email];
+		  [signerEmails addObject: email];
+		}
 	      X509_email_free(emlst);
+	      [signersEmails addObject: signerEmails];
 
 	      memset(p, 0, 1024);
 	      buf = BIO_new(BIO_s_mem());
@@ -227,6 +237,15 @@
         {
           validationMessage = [self labelForKey: @"Message is signed but the certificate doesn't match the sender email address"];
         }
+
+      // Every signer certificate must be issued for the sender address
+      if ([senderAddress length] && [signersEmails count])
+        {
+          signersMatchSender = YES;
+          for (i = 0; i < [signersEmails count]; i++)
+            if (![[signersEmails objectAtIndex: i] containsObject: senderAddress])
+              signersMatchSender = NO;
+        }
     }
   else if (!validationMessage)
     {
@@ -244,6 +263,14 @@
   return validSignature;
 }
 
+- (BOOL) signersMatchSender
+{
+  if (!processed)
+    [self _processMessage];
+
+  return signersMatchSender;
+}
+
 - (NSDictionary *) certificateForSubject: (NSString *) subject
                                andIssuer: (NSString *) issuer
                               withEmails: (NSArray *) emails
@@ -283,6 +310,11 @@
   return NO;
 }
 
+- (BOOL) signersMatchSender
+{
+  return NO;
+}
+
 - (NSString *) validationMessage
 {
   return nil;
diff --git a/UI/MailerUI/UIxMailView.m b/UI/MailerUI/UIxMailView.m
index 99660edc9..28587ef93 100644
--- a/UI/MailerUI/UIxMailView.m
+++ b/UI/MailerUI/UIxMailView.m
@@ -54,6 +54,7 @@
 #import <MailPartViewers/UIxMailRenderingContext.h> // cyclic
 #import <MailPartViewers/UIxMailSizeFormatter.h>
 #import <MailPartViewers/UIxMailPartViewer.h>
+#import <MailPartViewers/UIxMailPartSignedViewer.h>
 
 #import "WOContext+UIxMailer.h"
 #import "UIxMailFormatter.h"
@@ -244,11 +245,16 @@ static NSString *mailETag = nil;
 
   // If we are looking at a S/MIME signed mail which wasn't sent
   // by our actual active user, we update the certificate of that
-  // sender in the user's address book
+  // sender in the user's address book. Only do so if the signature
+  // is valid (trusted chain) and every signer certificate is issued
+  // for the sender address.
   from = [[[[self clientObject] fromEnvelopeAddresses] lastObject] baseEMail];
 
   if (![[context activeUser] hasEmail: from] &&
-      [[self clientObject] isSigned])
+      [[self clientObject] isSigned] &&
+      [viewer isKindOfClass: NSClassFromString(@"UIxMailPartSignedViewer")] &&
+      [viewer validSignature] &&
+      [viewer signersMatchSender])
     {
       SOGoContactFolders *contactFolders;
       NSData *p7s;