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I already have this feature, although it is using a commercial script called Hive Mail:
Link to Hive MailI have custom scripts that allows blocks in XOOPS to show members of new mails, what in there other box's, this webmail program also has calanders and much more, they allow pipe E-mail and also catch all accounts, so if you host allows a E-mail account to catch all, the program then deals with forwarding it to the right members. one a user signs on to XOOPS they get a E-mail account by simply logging into the mail script.
My Mods do make changes to the core of Xoops, and the E-mail works seperate to xoops, although they use the same userbase, like a multi log in, so you would have a XOOPS site and E-mail site, using the same info in sync with a full XOOPS block.
Because it's commercial you would need to purchase it from them, I would then be willing to sell this module on. (And no the software is not GPL, although I have it intergrated into Xoops, so the code cannot be re-distibuted for free).
This option is open to you.