Can't see 'sent' folder on an IMAP account
I've migrated a cPanel virtual server across to my Webmin server.
It appears to have gone well, aside from one email account, which now cannot show its 'sent' folder in Outlook or Thunderbird. The same problem occurs in webmail.
Thunderbird and webmail both advise looking at the error logs for the time and date of notification.
My server is running Postfix and Dovecot.
My questions are:
1. Which log will likely contain information about the error and where can I find it?
2. Is there a way I can verify that a mail account's folders or check that they aren't 'corrupted' in any way?