If I have a client who pays their bills, spends lots of money, and sends a few emails here and there I won't bill for those emails unless there is a distinct task involved.
If I have a client who pays their bills but doesn't spend much and they send occasional emails, I'll usually answer them for free but if it's excessive I'll send an invoice for 15 minutes (minimum billing).
If I have a client who just sends emails, I send them an invoice for actual time.
These problems usually self-correct. A good client will respect your time.