Handle, in the `rcptto_list` method, a mixture of recipient statuses.

[zdAlexM]

Hi,

I've found an issue with the rcptto_list method in Net::SMTP. It attempts to protect against "unknown users", but only tests against 53x-class errors, which are authentication-related. Additionally, in the case of some acceptable recipients, but others returning 53x-class errors, the block would be yielded to before the exception is thrown, potentially sending messages to the accepted recipients.

This patch updates this library to be more robust against mixed SMTP statuses.

• A new exception, SMTPMailboxPermanentlyUnavailable, can be thrown from any method that accepts a recipient address. It is a subclass of SMTPFatalError.
• A new exception, SMTPMixedRecipientStatus, can be thrown from rcptto_list. It is a subclass of SMTPMailboxPermanentlyUnavailable.
• In the event of all recipients being rejected by the server, previous versions of this gem would raise an ArgumentError that was indistinguishable from providing 0 recipient addresses. Now it will raise a more specific SMTP error.
• These exceptions are thrown before the method yields to the block. This allows the caller to inspect the exception and determine whether or not to continue with the transaction, instead.