Email is an addressing system and message format by which messages can be sent between users over the internet.
ISPs provide internet service. Sometimes they also provide web or email services over the internet as an add-on, and sometimes they don't.
It is quite possible to send and receive email messages without one's ISP even being aware of the fact. Indeed, most people do. If you have a large site, you probably run your own email servers. You emails go over your ISP's internet service, but do not use your ISP's email service, even if it has one.
Conversely, if you use webmail, your email does not reach your network in the form of messages - only web pages. Your messages originate or terminate with your webmail provider, who may well not even be in this country.
Only if you use the old-fashioned POP3+SMTP setup, or your ISP's webmail service, will your ISP see your email as email. In some cases it might be possible for them, by searching your entire network traffic, to identify and extract email from your network flow. That involves a whole lot of processing that they would otherwise not need to do.
If you use an offshore webmail provider, they can't even do that, because the traffic between you and the webmail provider is encrypted.