fix(mail): sanitize the domain part of generated message-ids

generateMessageID: appended the substring following the last '@'
without sanitizing it, so a sender value such as 'Doe, John <a@b>'
produced 'Message-Id: <uuid@b>>' with a duplicated closing bracket on
calendar invitations. A nil domain also left the message-id
unterminated.

The domain part is now cut at the first address delimiter or
whitespace - a domain is a single token, and anything else would
allow header injection through generated message-ids.

Fixes #6201

SoObjects/Mailer/NSString+Mail.m

@@ -564,7 +564,7 @@
 {
   NSMutableString *messageID;
   NSString *_domain;
-  NSRange r;
+  NSRange r, cutRange;
 
   messageID = [NSMutableString string];
   [messageID appendFormat: @"<%@", [SOGoObject mailUniqueMessageId]];
@@ -578,9 +578,15 @@
     }
     else
       _domain = mailOrDomain;
-    [messageID appendFormat: @"@%@>", _domain];
+    _domain = [[_domain componentsSeparatedByString: @">"] objectAtIndex: 0];
+    cutRange = [_domain rangeOfCharacterFromSet:
+                  [NSCharacterSet whitespaceAndNewlineCharacterSet]];
+    if (cutRange.location != NSNotFound)
+      _domain = [_domain substringToIndex: cutRange.location];
+    if ([_domain length] > 0)
+      [messageID appendFormat: @"@%@", _domain];
   }
-    
+  [messageID appendString: @">"];
 
   return [messageID lowercaseString];
 }