Sender is throttled error message

"Sender is throttled" is returned when you try to send email via your SMTP server.  
 

Where can I see how many emails then domain is sending ?
Login to your postmaster@domain.com via http://mail.domain.com, then go to Report ->  Domain Summary Report -> Traffic Report -> Message Traffic.

You can also find the how many email messages were sent by a single email account hourly from postmaster user -> reports -> domain summary reports -> message traffic -> select mail user -> select "hourly" under Group -> Generate Report.

To prevent spammers from abusing our email server, we place a limit of 50 emails per hour.  If your application need to send more than that, please consider to purchase more sent capacity from add-on products (Sent-Limit100).

If you purchased sent-limit100, and want to use one single email account to send more than 50 emails per hour, Please go to webmail with postmaster -> settings -> domain settings ->users -> throttling -> adjust the value.