I'm afraid problem of sending email is not a problem inside dolibarr but with your infrastructure setup. So it is not possible for me to help you more.
If you still have problem to setup your environment, i can recommand to use a ready to use hosted version, so you will benefit support of teams that can tune infrastructure for you.
You may find list of such services here:
http://wiki.dolibarr.org/index.php/Cloud_Solutions
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