We often run into issues where the default settings on Exchange, 2010 is set to only allow 5 emails at the same time from a user. As our software often exceeds that (typical error is submission rate failure) we suggest you increase that to 100.
This is the power shell command to run in exchange 2010 to make this change:
Set-receiveconnector "[relay connector name] [server name]" -Messageratelimit 100
This should allow up to 100 messages to be sent from the same user name in a time out period.
Due to heightened security in 2010 exchange, there is a low limit set by default to help with spam issues.