I saw this was discussed a month or two ago, but no conclusion seemed to be reached.
I have qmail 1.03, and vpopmail 5.4 installed, and everything works except for SMTP AUTH. I get segfaults or bad user errors, depending on whether or not vchkpw is owned by root or by the vpopmail user. Here's a transcript from a session where vchkpw was owned by vpopmail. When it's owned by root, the last line changes to simply say the authentication failed. 220 domain.com ESMTP EHLO domain.net 250-domain.com 250-AUTH LOGIN PLAIN 250-AUTH=LOGIN PLAIN 250-SIZE 0 250-PIPELINING 250 8BITMIME AUTH LOGIN 334 VXNlcm5hbWU6 dXNlcm5hbWVAZG9tYWluLmNvbQ== 334 UGFzc3dvcmQ6 cGFzc3dvcmQ= 454 oops, problem with child and I can't auth (#4.3.0)
