Not receiving emails - Sent emails not being received

Business contacts that I have emailed and they have emailed us many many times are NOT receiving my emails.  Also, we are now only receiving a few (if that) of emails that people are sending to us.  I have checked all our settings (spam, forwarding, etc) and they are all right but we still have issues.  This has been going on for about 2 weeks now and we can't run a business like this.  We rely heavily on email communication.

Need a fix ASAP