Among other things, my softlimit was set too low. I bumped 2000000 to 3000000 and everything seemed to work fine. For other googlers I discovered what was the matter with a telnet session:
$ telnet cono.org 220 vps1.digitalfeet.org ESMTP
250-vps1.digitalfeet.org 250-AUTH LOGIN CRAM-MD5 PLAIN 250-AUTH=LOGIN CRAM-MD5 PLAIN 250-PIPELINING 250 8BITMIME
> AUTH LOGIN
> <username -- base 64 encoded>
> <password -- base 64 encoded>
/home/vpopmail/bin/vchkpw: error while loading shared libraries: libc.so.6: failed to map segment from shared object: Cannot allocate memory 535 authorization failed (#5.7.0)
Unfortunately only the 530 authorization failed message shows up in the client so one does need to telnet to see the actual problem. Also note that one doesn't actually need to base64 encode the username and password in order to generate this error, but ordinarily one would use the encoding to transmit the username and password.
This was discussed earlier in the list in a thread titled: "Help!! with qmail+vpopmail PLEASE!!!". I should have read on suppressing my aversion to such nondescript subject lines--though I suppose my own isn't anymore helpful, now that I think about it.
John-Mason P. Shackelford -- http://john-mason.shackelford.org