Extremely slow - like hours slow

Extremely slow email sending, receiving and forwarding.  I sent myself a test email and it took a very long time to come through.  I sent my self a test email to my home email (cox.net) and it went through in seconds.  Some of my customers are calling and saying I sent you an email and it doesn't show up until hours later.  I have all my emails from your server forwarded to yahoo (for storage reasons) and I work from that platform.  I check the open exchange server everyday and when I see that all the same emails have shown up at my forwarded yahoo acct, I delete on open exchange so I will have plenty of space on there.  The emails are showing up on open exchange several minutes after I send a test and then it takes 30 minutes and up to several hours to forward to Yahoo.  Why do you think this is happening?