This is a big problem. 

When replying to a Customers ticket, a vast number of Gmail and Yahoo users are not receiving our replies, they can not locate in any of their folders including Spam/Trash. They also can not locate us by using search through all of the folders.  Why is this and what can be done about it?

An even bigger problem is that we are not receiving some of our customers replies to ongoing threads. They are able to supply the screen shot fro their end showing that their reply was sent however at our end this does not show and only shows our response as being the last correspondence. 

Please do not ask me for error messages as there are not any, 

This is urgent, customers need to have a reliable communication method. Primary Contact