Migrating email advice

Hi, hope I’m posting this in the right place!

Some advice please - I’ve been moving some websites from a Centos CPanel based server to a Ubuntu Virtualmin based one. All is going well, except that the email account users are reporting multiple copies of mail coming in. It appears to be mail from the last day or two that the previous server was in use and it’s not just one or two copies - it’s sometime hundreds!

I can understand how a single duplicate mail could occur - the client’s email software thinks it’s a new unread mail because it’s coming from a new server - but I’m not sure why there’s so many copies coming in. The MX DNS record is simple and definitely correct. And the account has been removed from the old server completely.

Is this something that can be fixed, or avoided for future migrations somehow? Thanks in advance.