E-Mails issues. The E-mail came late.

Question

I have issues with mail yesterday. The mail is being received a day later with the last day date.
The algoritm it is like this
1. Change the process of the shipment products
   2. For x = 0; x < shipment products; product++
        3. Sent email "Product[x] has been changed to process" (to 5 e-mails)
Last shipment has like ten products  for each product an email was sent and the software did not catch any exception at all.
So this problem is from the Mail Server? Am I charging the mail server with a lot request?