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I think you can use the sqmail login to autologin the XOOPS account, the problem is that your POP/IMAP email account name and pass can be different from Xoops', a XOOPS user can have multiple email accounts or none at all!
In current XOOPS implementation there is no info about the mapping of Xoopsuser-EmailUser, so the mail module should also maintain the mapping. In short, integration is possible provided hacking either XOOPS core and sqMail.
wjue