Allow S/MIME signature for all mail methods

The original implementation only enabled S/MIME signature for
PHPMAILER_METHOD_SMTP.

Moving the call to PHPMailer::sign() out of the switch() statement, so
it is also called when mail method is PHPMAILER_METHOD_SENDMAIL or
PHPMAILER_METHOD_MAIL.

Fixes #25764

core/email_api.php

@@ -1332,16 +1332,17 @@ function email_send( EmailData $p_email_data ) {
 
 			$t_mail->Port = config_get( 'smtp_port' );
 
-			$t_mail->sign(
-				config_get_global( 'email_smime_cert_file' ),
-				config_get_global( 'email_smime_key_file' ),
-				config_get_global( 'email_smime_key_password' ),
-				config_get_global( 'email_smime_extracerts_file' )
-			);
-
 			break;
 	}
 
+	# S/MIME signature
+	$t_mail->sign(
+		config_get_global( 'email_smime_cert_file' ),
+		config_get_global( 'email_smime_key_file' ),
+		config_get_global( 'email_smime_key_password' ),
+		config_get_global( 'email_smime_extracerts_file' )
+	);
+
 	#apply DKIM settings
 	if( config_get( 'email_dkim_enable' ) ) {
 		$t_mail->DKIM_domain = config_get( 'email_dkim_domain' );