As the imports are optional, the OSGi resolver can choose the drop the imports, if no exporting bundle is available, or if it can't wire to the exports for some reason.
During startup, dm Server's provisioning support will attempt to satisfy every optional import, i.e. when the context support bundle is installed, dm Server will detect that imports the javax.mail package, and will look in its repository for a bundle which exports this package. If such a bundle is found, it will be installed, and the resolver should then wire the context support bundle to the javax.mail bundle.
The only way that you can force a import to be wired is by making it mandatory, i.e. removing resolution:=optional from the import. In this case, if the import can't be wired to an export, the importing bundle will fail to resolve.
It would be interesting to know if the javax.mail bundle gets installed, but the context.support bundle doesn't wire to it, or if dm Server doesn't install the bundle at all.