add additional mail_smtp.. parameters to fix possible SMTP connection problems.

Added three additional mail_smtp.. parameters.
- mail_smtpdebug - enable debug messages to analyse SMTP problems.
- mail_smtptimeout - set SMTP timeout which is set to 10s by
  default and this is sometimes to short especially if a malware/
  spam scanner is used.
- mail_smtpsecure - force secure SMTP connections.