Some business contacts have reported their emails sent to me are not going through

Some business contacts have reported their emails sent to me are not going through

However, they do not get a "hard" failure message. So far the only three business associates that have reported a problem are both located in the province of Ontario, Canada. Is there any chance any of your mail servers may have been blacklisted by any Ontario based ISPs such as Bell or Rogers?

I receive emails from other areas in Canada and from around the world with no apparent problems. Sending emails always works.