I just picked up a new customer who transferred from another host. I duplicated their entire web site and the email accounts on our server, and then submitted the request to NSI to update the name servers. The DNS propogated about 2 weeks ago, but certain emails still arrive on the old server. The emails are all from the same person. Is this a caching issue from the email sender's ISP? Thanks in advance for your help!

If the dns/mx records have in fact been changed and propogated, then it's likely the person in question:

1) Is using the same mail server as the old host and it still has the mail account on it. Mail servers look for addresses in "local" domains (without ns lookup) first, so any user on that server will be unable to send mail to the actual host until the local host removes it.

2) Has a system, name server or proxy with static or cached hosts including that mail server's dns. This could be the case with members of the isp who initially setup the old, since they'll often alter their DNS records to test the accounts. This could also be the case with a local ISP who's DNS records aren't updated for some reason, though after two weeks, that's pretty unlikely.

As above. Perhaps you could request for the client to request the old web host for the DNS entries be removed.

Thanks for the help! I think these suggestions should take care of it...

I had the same problem while transfering a few of my client acounts over to a new server. After removing DNS entries from the old server things started working fine.