Hi, I'm looking for a mail client that is well suited for managing multiple identities and can easily handle routing everything over an anonymity network.

I would use Thunderbird, but I think when you take it online, it downloads from all your connected email accounts. I want to "go online" at will toward particular email addresses, in other words I do not want my upstream mail provider to be able to associate my accounts in any way, including access time, assuming there is a large enough other pool of people using the same client/anonymity network.

Are there any that are well made for this purpose? Otherwise I will use the mail frontend over Tor or something, but it would be nice to have a lightweight client-side application too so I can keep my emails downloaded and delete them from the server.