Send E-Mail failed to send mail

Hi,

I have a requirement to send about 1000 mails based on number of records updated. I am trying to send using MNI. I tested for 600 records at once and most of the process breaks and got error alters. I reduced the batch size to 150 then it works fine. But if the data is about 200 then the process starts to break. 

The error is Failed to send email.

Is there any limit to send mail.

Is there any other way to send mail efficiently.

Please let me know.

Thanks