They can be either desktop based or server based.
thanks for the link
Wow, i googled myself for a while but did not find that one....
SES does not use SMTP, but rather specialised API calls. The client you found is rather a gateway, that will accept SMTP and "output" the API calls.
This brings some limitations. With SMTP i need to "create" each message locally through some form of mail merge (I understand Thunderbird supports that). This means that I need to "create" each mail locally and then send them to the server. My hope that i would upload the message (formatted with the proper fields) plus a CSV and move the heavy lifting to the cloud.
I found a beta client developed by a user.
Though if it uses SMTP you should be able to use any client, such as Thunderbird.