Slow delivery on e-mails

[kaizer]

My clients are complaining about delayed delivery on their e-mails, it can take about an hour before the message are delivered to their mailboxes. Any suggestions on how to fix this?

Our server is running Debian 5 with Plesk 10.0.1 (with Qmail mailserver)

 

[zeroday]

double check if you have enabled greylisting, this can cause severe delay in delivery (but prevents spam)