As Paul said, if you have users who are working during your backup period, you can't really stop them from changing mail items. Remember that 'mailbox' backup is really backing up each mail message individually, as if it was a file. - so, not much different than someone changing a file while backup is running.
It's not really critical - it only refers to individual mail messages, not the whole mailbox.
Your real concern should probably be that your 'information store' backups are completing without errors. This is what flushes the transaction logs and what you will need if your whole Exchange system (or a storage group, or a single store) goes "belly-up".