I've been tracking down a problem when trying to use gmail's smtp server for relaying.

We are using debian and post fix.

Errors like the following appear:

certificate verification failed for smtp.gmail.com[74.125.47.109]:587: untrusted issuer /C=ZA/ST=Western Cape/L=Cape Town/O=Thawte Consulting cc/OU=Certification Services Division/CN=Thawte Premium Server CA/emailAddress=premium-server@thawte.com

warning: SASL authentication failure: No worthy mechs found

The fix on debian:

apt-get install libsasl2-modules

That's right.  When you have SASL installed on debian you also need to install the authentication mechanism modules.  Great.

I'll update this post with instructions on how to get your debian MTA server to relay to gmail.