Mention emails were not recieved on Gmail

[Przemysław Ferkaluk]

Hi, 

a co-worker informed me that he was mentioned three times in a task. He claims, he has not received email notifications for the first two mentions. However, there was a third notification in his mailbox along with the two previous ones - as separate messages in one thread regarding this task.

Is there any way to tell what happened to the two previous messages? Why didn't he received them? Are logs of outgoing emails available?