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We're currently developing a webmail module which could probably fit your needs: it is called WebMailFX. The module is still in a developing phase but it promises to be a good one. All in all, it's probably the only one webmail module for xoops, if we except the SquirrelMail port.
Wer have a project on dev.xoops.org. You'll find more infos there:
http://dev.xoops.org/modules/xfmod/project/?group_id=1132
Bye,
ft