OpenEMM Support

I'm testing OpenEMM 5.0.2 on Slackware 11.
I tried a couple of times to send a newsltter to 4 email addresses and it work.

But later then, when i send the 6th mailing, the delivery delays for 3 hours, i didn't change config of openemm and sendmail, any direction for me to solve it ? What i need to ...