mail() function inquiry..

[neo777ph]

Hi..I had this problem since last week i can't find a solution..
i used the Mail() function for sending mail on my PHP built Internal system.
My mail sending works intermittently..ON & OFF -- I want it to be consistent in mail sending..
though it does not show an error message that the message was not sent,,so i assuming that it was sent.
However, when i had read the PHP.net documentation on the mail function it states that :

"Mail Function
Return Values
Returns TRUE if the mail was successfully accepted for delivery, FALSE otherwise.

It is important to note that just because the mail was accepted for delivery, [color=red]it does NOT mean the mail will actually reach the intended destination.[/color] "

My Question:
given this statement how can i make sure the mail is always received by my recipients?

FACTS:
My mail server is an Exchange Server located at malaysia..I am at (webserver@Philippines).
There are two specific users of my system that cannot sent mails on each other via my PHP system.
PHP TRIAD -- PHP 5.0 Apache 2.0 MySQL Db..
Spec.. 3.0 GHz Celeron and 512 MB memory..

Please I would appreciate your help! Thanks a lot!