In Universal Email 3, the maximum emails that can be sent is determined by the SMTP Server settings.
The SMTP Server will have a setting for the maximum number of email it can accept in a given time period.
Universal Email 4 has settings available to set the number of emails that it will send in a given time period, and the amount of time it will wait between sending emails to get around these server limitations. this is not available in UE 3 however.