SOLUTION TO THIS PROBLEM:
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The fix was given to me at the Sun forums, in the JavaMail discussion section.
So I want to share it with you in case someone else runs into the same problem.
The UnrecoverableKeyException is a manifestation of:
https://glassfish.dev.java.net/issues/show_bug.cgi?id=6938
It is possible to work around this by adding two lines to the domain.xml of the application running on GlassFish.
1. Go to the following directory: <glassfish_folder>/domains/<your_domain>/config
2. Open 'domain.xml'
3. Look for the <java-config> section
4. Add the following lines:
<jvm-options>-Djavax.net.ssl.keyStorePassword=changeit</jvm-options>
<jvm-options>-Djavax.net.ssl.trustStorePassword=changeit</jvm-options>
5. Restart GlassFish
Sending emails through ports 587 and 465 should work now within a https-loaded page.
This applies to Grails Mail Plugin too.
Thanks everyone for your help.
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