Cannot send email to Symantec email clients

We have several customers that use Symantec email services. We've been unable to send email to these customers for over a month. Our IP does not have a negative reputation. Any help would be appreciated!

Sending IP: 96.27.203.122

Sending domain: perfhyd.com

Error message:

Attempting cluster6.us.messagelabs.com:25...
Trying to connect to host cluster6.us.messagelabs.com...
Trying to connect to TCP/IP address 216.82.251.38 on port 25.
Trying to connect to TCP/IP address 216.82.241.100 on port 25.
Trying to connect to TCP/IP address 216.82.242.36 on port 25.
Trying to connect to TCP/IP address 216.82.242.46 on port 25.
Trying to connect to TCP/IP address 216.82.249.212 on port 25.
ERROR: It was not possible to connect.

Thanks!