I am a webhost and am planning on moving my customers to a new server (upgraded version of ensim + RHEL). Now, I will move their accounts and then change DNS to reflect the new server. However, my question is, how is it possible to for customers to retrieve e-mail on the old server, after the dns changes have resolved. Any e-mails not checked before I changed DNS will not be downloaded by the customer, and won't e downloaded by the customer.
I am thinking of perhaps copying e-mail account files to the new server, and merging them into the new server's email files. Is this even possible with linux? If so, can you give me the command to "merge" two files?