SMTPProvider should check server's capabilities before using them

[fs]

The !SMTPProvider should check if EHLO/TLS is supported before issuing these commands. If the user enabled TLS but the server does not support it, an exception may be raised.

[amcgregor]

AFIK the EHLO statement is what allows the SMTP client to determine the capabilities of the server. The TLS option was defined as a three-state switch:

• False, do not use TLS.
• None, (no decision) automatically determine TLS capability and use if available.
• True, force the use of TLS even if not advertised.

This is how it is currently configured. I will admit, however, that the conditional blocks in source:trunk/turbomail/providers/smtp.py#L48 aren't the most obvious and should probably be cleaned up. I'll close this ticket as invalid for now. If additional information is brought to light (about how to determine if EHLO is available), it should be re-opened.