diff --git a/api/src/Mail.php b/api/src/Mail.php
index 3b2c11ba2e..cd0da046cf 100644
--- a/api/src/Mail.php
+++ b/api/src/Mail.php
@@ -7608,7 +7608,7 @@ class Mail
 
 		$params  = array (
 			'type'      => 'message',
-			'pubkey'    => $certkey[$acc_smime['acc_smime_username']],
+			'pubkey'    => $certkey[strtolower($acc_smime['acc_smime_username'])],
 			'privkey'   => $acc_smime['pkey'],
 			'passphrase'=> $_passphrase
 		);
diff --git a/mail/inc/class.mail_compose.inc.php b/mail/inc/class.mail_compose.inc.php
index 167ab82152..99c54b4a4d 100644
--- a/mail/inc/class.mail_compose.inc.php
+++ b/mail/inc/class.mail_compose.inc.php
@@ -3829,7 +3829,7 @@ class mail_compose
 		{
 			$sender_cert = $AB->get_smime_keys($sender);
 			if (!$sender_cert)	throw new Exception("S/MIME Encryption failed because no certificate has been found for sender address: " . $sender);
-			$params['senderPubKey'] = $sender_cert[$sender];
+			$params['senderPubKey'] = $sender_cert[strtolower($sender)];
 
 			if (isset($sender) && ($type == Mail\Smime::TYPE_SIGN || $type == Mail\Smime::TYPE_SIGN_ENCRYPT))
 			{
@@ -3843,7 +3843,7 @@ class mail_compose
 				$params['recipientsCerts'] = $AB->get_smime_keys($recipients);
 				foreach ($recipients as &$recipient)
 				{
-					if (!$params['recipientsCerts'][$recipient]) $missingCerts []= $recipient;
+					if (!$params['recipientsCerts'][strtolower($recipient)]) $missingCerts []= $recipient;
 				}
 				if (is_array($missingCerts)) throw new Exception ('S/MIME Encryption failed because no certificate has been found for following addresses: '. implode ('|', $missingCerts));
 			}