We are sending hell lot of e-mails to our BREW devices ( in sprint network ) and after a while our mail server queues the messages and gets stuck. When we try to flush them,
This is called grey-listing. When you send too many e-mails (or more often - a certain number of e-mails where a recipient does not exist) a destination mail server does not black list you, but instead they temporarily block access from your mail server (essentially the IP address of your mail server). Usually this block is set for 1 hour but obviously can vary depending on the configuration.
You can do several things: