Ebuild uses virtual/jdk-1.5 as a dependency. However, JavaMail can run under 1.4. Since 1.5 is not perfectly backwards compatible, I'd suggest it be changed from 1.5 to 1.4 in the eBuild.
From JavaMail 1.4's readme:
>JDK Version notes
>-----------------
>
>The JavaMail API supports JDK 1.4 or higher. Note that we have
>currently tested this implementation only with JDK 1.4 and 1.5.
>While JavaMail will work with JAF 1.0.2, we recommend the use of
>JAF 1.1.
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From JavaMail 1.4's readme:
>JDK Version notes
>-----------------
>
>The JavaMail API supports JDK 1.4 or higher. Note that we have
>currently tested this implementation only with JDK 1.4 and 1.5.
>While JavaMail will work with JAF 1.0.2, we recommend the use of
>JAF 1.1.