gh-XXXXX: Fix smtplib.quoteaddr() returning malformed address for '<'

Input starting with '<' but missing closing '>' was returned verbatim.
Ensure the result always ends with '>'.

Author: stefanzetzsche

Lib/smtplib.py

@@ -150,7 +150,9 @@ def quoteaddr(addrstring):
     if (displayname, addr) == ('', ''):
         # parseaddr couldn't parse it, use it as is and hope for the best.
         if addrstring.strip().startswith('<'):
-            return addrstring
+            if addrstring.strip().endswith('>'):
+                return addrstring
+            return addrstring.strip() + '>'
         return "<%s>" % addrstring
     return "<%s>" % addr