Got it !!!
Ok, for the record, I'm using PHP 5.2.6 in a Windows box. This version of PHP has a known bug in the mail
The bug was fixed in 5.2.11 and in 5.3.x branches.
Though actually, if you read the discussion there, they treat it as a workaround and not a bug fix. According to RFC2821, the RCPT TO field in an e-mail message must be e-mail only:http://tools.ietf.org/html/rfc2821#section-6
The bug patch includes a patch for PHP that prevents the mail() call (which is used by PHPMailer, which in turn is used by Wordpress, which is used by s2Member) from enveloping the address with "<" and ">".
Anyone with a Windows box (pre-5.2.11) to confirm? Maybe a workaround is in order?
I think I can workaround it with the "wp_mail" filter, but this might bug other Windows users with older PHP versions (lower PHP 5.2 versions are still very common I think)