I want to move just one client account from a Plesk server to another Plesk server? Which files should I backup (in the old server) and restore (in the new server)?

I want to keep the site and mail configuration in the moved account.

Thanks in advance!

Since it is only one site then its better create the account in the PSA(newserver) then transfer the files.

you need vhost files from your old account, then you should get moved the mails, databases as well.

if you want to create the mail accounts in the new servers. view the database PSA> mail> you can find the username and passwords for all your mail accounts.