Cannot delete smtp server in mail application on High Sierra
I have found in Keychain two hidden entries that corresponded to smtp servers (com.apple.account.SMTP.asynchost), delete them and puff, no more ghost smtp server on the mail app.

Take caution as deleting those entry apparently can lead to keychain malfunction. Do backups of the keys before delete them.

See discussion here: https://discussions.apple.com/message/32426759#32426759