Tools/Options/SSL
online: http://www.stunnel.org
QS uses the highly regarded open-source software, Stunnel, to provide TLS/SSL connectivity with services such as gmail and yahoo. If you're wondering, the name Stunnel is derived from secure tunnel, a metaphor for hiding the data you transmit to the mail server.
Stunnel can provide a general purpose SSL proxy that your computer can use for various purposes where encrypted connections are required. However, QS doesn't setup Stunnel in that manner. It loads Stunnel for only the few seconds required for the mail transfer, then immediately unloads it. From your perspective, stunnel functions seamlessly with QS.
If you have Stunnel already installed on your computer, that's not a problem. QS Stunnel will still work just fine anyway--or, you have the option of using the previously installed Stunnel if it provides the requires service.
Listen Address
This is the address Stunnel listens on. It is normally 127.0.0.1 and I doubt you'll ever need to change this.
Accept Port
This is the specific port stunnel listens on. I chose the value more or less at random and, because of the great range of port numbers, the chances of it conflicting with an already installed Stunnel are very unlikely. If it does conflict, a change to any other port will do. I'd select something between 1000 and 9999.
Use Certificate
An SSL Certificate is a document that describes who you are. Though SSL servers absolutely must provide certificates, clients--such as QS--usually don't." On rare occasions, a server will be configured to check to see if a client has a certificate. On even rarer occasions the server may require the certificate meet certain requirements.
Stunnel comes with a demo certificate for test purposes. It's called stunnel.pem. If a service complains that you haven't presented a valid certificate, tick the Use Certificate box and stunnel will use the test pem that's included. That'll probably solve your problems. If the service continues to complain that the certificate isn't satisfactory, you'll need to purchase a certificate from a service such as VeriSign--which is very expensive--or generate your own self-signed certificate for free. Here's is an online tutorial that can help you out with that: online: How to create a self-signed SSL Certificate
If you have to get a new certificate, call it stunnel.pem and put it in the QS home directory.
All that having been said, you won't need a certificate.
Keep Log
There is little need to keep a stunnel log. However in the unlikely case that you are having trouble with stunnel setup, enabling logging can be helpful. The log is named stunnel.log and like stunnel.pem is stored in the QS home directory.
Delay DNS Lookup
Delay DNS lookup until connect time. This appears to be useful if connecting to a service whose ip is dynamic and might change between sessions. For example, a service provided via dyndns.com
Further Info
For those of a technical sort, who want to know a bit more about how QS uses Stunnel, read this topic, Stunnel Configuration, from the help Appendix.