Mail from VPS server has been failing to comcast.net email addresses.

Hello, I am the server administrator for a retailer who uses a VPS server to host email services and has recently been experiencing the following error when sending to comcast clients: 

all hosts for 'comcast.net' have been failing for a long time (and retry time not reached)

We suspect that the VPS provider smarthost has been blocked and would like to confirm this before approaching them, yet again, with this issue.  

Regards,

Lawrence Mitchell

Blue Star Performance