Forum OpenACS Development: Re: Naviserver on Windows - "select failed" when using ns_sockselect
Btw, also back then, I'd previously seen ns_sendmail work just fine on Windows XP running on VMware Workstation, but then fail every time after we switched to running XP in VMware Server on a different physical machine. Why, I do not know.
the change to use acs_mail_lite::send (and therefore using tcllib's smtp::* APIby default) instead of ns_sendmail happened 7 years ago . in case, the error comes from tcllib, maybe  can help to debug.