Exchange 2013 - Cant connect on site non domain PCs to exchange, but can off site?

[burgemaster]

Can anyone please help us out here...

Exchange 2013 CU7

 

Computers in AD - Autodiscover works perfectly, email pre-entered, next, ok and there all set as normal

Home computers - Email and passwords manually entered, one windows authentication pop-up and all set.

On Premises workgroup PCs - email and password entered, we get two of the three green ticks and then "The name could not be matched to a name in the address list" ?

 

When testing with exchange connectivity tests all is fine. We have split DNS setup and all entries entered point to the exchange server.

Can anyone please suggest what could be causing this or where we should start to look?? were stumped!

 

Thanks in advance