I have begun to experience a failure of the receipt of emails. Some come through okay, but I know that all are not coming through, as I have certain messages that come to me each night, and they are not appearing. I have sent test messages to myself, and some come and some never appear. I have checked with my email provider, and their system appears okay. I cannot figure out where to look for the problem. I am using Thunderbird, a DSL connection via Optimum Online and RCN as email provider. TIA for any suggestions.