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Is it essential that it's a XOOPS module?
I'm not sure if I'm allowed to post this as it's non-XOOPS related, but there is another project called RouncCube, I really like their work, it is of course a webmail project:
http://roundcube.net
It's GPL also.
I use this for my E-mail users, it's more efficient for me, and I've never had issues with it so far, but it depends on what your looking for.
Hope this maybe of some help.
If your looking for something integrated with the one login for XOOPS, then I'm afraid I think the only solutions are those you've already tried.
Kind Regards.
Tom
http://bassmanthemes.com
http://www.xoopslance.com