SMTP emails sending intermittent

We are having endless problems sending emails from our network. Sometimes it will send just fine but minutes later email sending will fail with an error connecting to SMTP. Through Network utilities on my iMac I can Ping the SMTP server and also rund traceroute, however the port scan fails on 587, 25, 495 and 2525. When the emails work, portscan also submits.
Can someone point me in the right direction here? Is the Peplink failing? We are using a Peplink Balance 710