We're set up on the online Exchange server for our organization and have been for almost 2 years. Everything appears to be running fine except we've now received reports from a few of our customers that they can receive our emails but when they "reply" or send us a new email from their accounts we do not receive them. They do not receive undelivered email notices on their end but they don't show up on our end. One of the companies is actually our accountant and we tried to trouble shoot it with them a bit but we can't locate the variable that might be causing their emails to not come through on our end. We send them an email. It shows up on their end. They reply from their end and it doesn't show up on our end. They've checked their spam settings and blocked email list and none of our email accts are blocked on their end. I've checked our spam settings and blocked email list and none of their accts are blocked on our end. They are able to send emails through to my own personal account that is not on our exchange server. I have 3 other customers who have reported this problem to us and who knows how many others that have not reported it.
Where else could I try and locate the source of this issue?