Mail not forwarded at the same time for many users

Some of my users remark that they did not received the same mail at the same time. Causing many "Didn't you read the mail?" situation...

What explain that the same mail is not forwarded to the users at least in the same 15 minutes?

Here's a log example of what I mean :

15:54:10   Object: TEST   dst:user1@mydomain.com   src: donotreply@otherdomain.com
15:54:10   Object: TEST   dst:user2@mydomain.com   src: donotreply@otherdomain.com
15:54:09   Object: TEST   dst:user3@mydomain.com   src: donotreply@otherdomain.com
15:54:09   Object: TEST   dst:user4@mydomain.com   src: donotreply@otherdomain.com
15:14:09   Object: TEST   dst:user5@mydomain.com   src: donotreply@otherdomain.com
14:54:09   Object: TEST   dst:user6@mydomain.com   src: donotreply@otherdomain.com
14:43:18   Object: TEST   dst:user7@mydomain.com   src: donotreply@otherdomain.com
14:43:12   Object: TEST   dst:user8@mydomain.com   src: donotreply@otherdomain.com