Major Description
As CAPA response contains multiple lines as response its neede to call getAdditionalReply() to consume all lines. Beside this it would make sense to move capa() from ExtendedPOP3Client to POP3Client. If you want to keep it in ExtendedPOP3Client it really would make sense to add all constructors of POP3SClient to allow also the usage with STARTTLS and POP3S