If you are trying to send an e-mail in Outlook or Outlook Express and you’re hitting the wall with:
501 Too many syntax or protocol errors
then 99.9% that you have a single or double quotes around e-mails addresses of your recipients.
Purging it (using any method) should solve your problem instantly in most of the cases.
I used Notepad++ for this purpose:
The biggest stupidity in Outlook is that in most cases it does not report that such error occurred and tries to send the same message over and over again, when you’re certain that everyone has already received it.
So check closely each list of your recipients, if it does not contain single or double quotes and pure them with above method.