Sounds to me like you need to engage an IT specialist. Seriously. Based on that question you are in a sink or swim situation and you will sink. Adding and removing email addresses is usually something done by the system administrator either on the mail server or as a part of the domain hosting with the domain host. The procedure for doing so will be entirely dependent on the setup of your IT environment. Adding and removing mail accounts is NOT part of Thunderbird, Thunderbird adds pre-existing mail accounts so the user gets their mail from the mail server. In you case, that could be a local mail server, a cpanel arrangement with a domain host or something that comes with Google apps or Office365. Those are the four common arrangements, but really you need boots on the ground that can go through your setup and check everything. If the parting was acrimonious and you sending sending letters via a lawyer instead of just ringing sounds like if it was not it is now. There could be all sorts of issues in the network that you are not aware of. Perhaps contact others in your industry to determine who they use for their IT work as getting someone familiar with the industry will save you money in consulting fees.