Email to a comcast.net email account not getting through

I own a travel company.  I am trying to email a client with a comcast.net email address that I have corresponded with before. 

My email message bounced and I got this error message:

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

I realize now that other emails to comcast.net recently bounced.

Perhaps my address has been blocked as spam.  Can you help?  I do have several active clients with comcast-net addresses.  Thank you!