[Dovecot] Mails repopping

HI

I don't know if this is a dovecot issue or a client one. But as Dovecot is the most recent change, I'll start here.

I have a server that's been running Courier for about 6 years and in all that time I think I've only ever had 1 issues where an entire mail box was repopped by a webmail client. However, since moving to a new server and dovecot 4 weeks ago, I've now had the webmail client repop this account 4 times (there are about 230 mails in the account).

Is there a setting I need to tighten to prevent/remedy this? I have no idea if it's happening on other accounts, but this is one that I see. The format is maildir. There has been no changes to the webmail client.

Thanks.

Simon