SMTPClientSession: replace bare newlines in message content with CRLF.

[obiltschnig]

Since SMTP servers (specifically, Postfix since early 2024) are increasingly rejecting emails containing bare newlines in their content (which is not valid in SMTP) due to potential SMTP smuggling attacks, its worth discussing whether SMTPClientSession should automatically normalize all bare linefeeds in messages to CRLF sequences.

The proper place for this fix would be in the MailOutputStream class, which already deals with CRLF sequences.