Although the issue is not critical in XOOPS environment, we would suggest to switch to other methods than Sendmail, or to download quick fixes: fix for XOOPS 2.0* fix for XOOPS 2.2*
Note: official response and solutions are still being waited from phpmailer development group, and will be released in XOOPS 2.017 and XOOPS 2.25
Thank GIJOE, irmtfan, vaughan etc for reporting and solutions.