This is caused by a bad DNS configuration. Generally, any mail leaving the server and originating from a virtual domain is rewritten so it looks like it came from the main domain of the server.
This problem originates from a new sendmail behavior. At startup, sendmail scans all network interfaces (and IP aliases) and retrieves the name associated with their IP number. It assumes, from now on, that all those names are equivalent to the main domain of the server. Any email originating from one of those names will be masqueraded as coming from the main domain.
The DNS problem is simply that you have set the reverse mapping for an IP number so it points to a domain name instead of a host of this domain. Linuxconf will not allow this mistake to occur, as it never does a reverse mapping pointing to a domain name. Doing a DNS by hand does not prevent that though.
So make sure that all IP numbers associated with IP aliases point to a host of a domain, not to a domain itself. Restart your DNS and your sendmail and all will be fine.
Another solution is to enter the proper definition in /etc/hosts. vpop3d looks there first anyway. So if you have mail.some-domain.com pointing to the IP x.y.z.w, just enter this line in the /etc/hosts file of the server running vpop3d:
Yes. Check out http://vimap.sourceforge.net. It is a POP-3 and IMAP server derived from the imap package found on RedHat.