@DerSchlingel
The account's recovery email address is released and can be reused soon after the user deletes the account. I don't recall the exact time, but IIRC, they're deleted in about 10-15 minutes.
As @DoctorG said, when deleting an account all data is deleted. This includes the user's Vivaldi email address and all messages in the mailbox, as well as the forum, Vivaldi Social, and blog posts.
We do reserve the usernames to prevent reuse, though.