Wait for the banner message from the mail server before delivery

Wait until the banner message from the mail server is displayed before continuing. In the event of a successful connection to the mail server but no welcome message yet - because the server is slow to respond - the mail library would simply continue with the SMTP conversation resulting in a "554 SMTP synchronization error". This change loops until $line (the response from the mail server) is not false. Ref: http://www.faqs.org/rfcs/rfc2821.html, section 3.1.