When logging into your email, you notice some messages are no longer present.
When an email goes missing, it is often from a user moving the messages to the Trash folder or accessing the account with a mail client that downloads and deletes the mail from the server.
Please note that, by default, Dovecot doesn't log what happens to an email message once it has been delivered to a user's mailbox. The mail-log plugin must be enabled for Dovecot to log post-delivery activity.
If the messages have been moved to the Trash folder, they can be moved back into the Inbox. If the messages have been removed from the server, the account will have to be restored from a backup.