The error appears to be saying that you are trying to send through a network (the cellphone provider's) that is not the same as the network of the mail provider. This might happen if the server settings in TB are non-secure, e.g. the smtp is on port 25 with Connection security = none. Without knowing the details of this setup, you might consider adding your mail account to the phone so that sending and receiving are done on the same network (the cell provider's). Or, see if the issue is caused by non-secure settings imposed by the mail provider. If they don't offer secure access, I would get a secure account with one of the major providers such as gmail, outlook.com, gmx etc. I frequently do email on a laptop with TB connected to my phone's wifi hotspot, with no issues, using secure accounts.