combining /var/mail files

[steve%prd.co.uk@localhost]

During a change of ISPs this week, I had to run mail service on a
remote VPS, and access emails there.   There must be gotchas if I
simply append the mail files from the VPS to the existing
corresponding files on our usual mail machine now it's got it Internet
connection restored.  There are always gotchas.  Can someone identify
them for me?

Thanks,

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Steve Blinkhorn <steve%prd.co.uk@localhost>