That error could be caused by any number of things such as:
- a bad email address in the to
- exceeding the SMTP Server limits
- the SMTP server timing out
- PHP server timing out
and numerous others.
Most common would be the bad to address, or exceeding the SMTP server limits. You should check with your host to determine what the maximum number of email you can send out in a given time period are.
If the host has a limit of 1000 emails in a 24 hour period, you can easily exceed that by leaving the limit as 200 / second (you would hit the limit in 5 minutes)
in universal email there is a logging section which can be used to record which email addresses have been sent to, if you did not set up the logging, there isn't a way to tell who received the email.