As with MintPPC64 I have been testing the email clients that are available to us. I am not very impressed by SeaMail and SpiderMail in the sid environment. It often crashes and the programs cannot remember the email password for retrieving new mails. I can only really recommend sylpheed. I will remove SeaMail and SpiderMail from the preseed file so future installations will no longer ship with these email clients.
To install sylpheed in your system do:sudo apt install sylpheed