Random dropping email via php mail function [duplicate]

This question already has an answer here:

• PHP mail form doesn't complete sending e-mail 7 answers

I have a problem I use very plain and simple php mail function:

mail($to, $subject, $message, null,'-fsmsmsmsms@smsmsms.smsm'); 

the problem is that this code is executed sometimes not correctly. Some friends are trying to send it from a pc and there is no error and the email is not send. I've made txt document to add to every subject and the integer in it is incrementing but the mail is not received. What can be the problem. From mine pc and the pc of some other friends there is no problem.

Best regards.