So if they do not open my email editor it is not possible for them to contact me unless I provided my address and I would, therefore, be unable to know, for certain, if they had received the email.
If it does rely on my email address then the one used will, presumably, be the address linked to my ISP. and so my email should appear in the sent column?
if when you use a 'Contact Us' button (or similar) from a website it gives you a form to fill then it's not using, doesn't need to use, your email.
(It sends the form directly from the website without even touching your email account).
If however it does need use your email then either:
It will open your email editor that your browser app is set to use;
Or if you haven't set which email to use it will give you an error message, or box to chose one.
This link shows an example of how this is set up in fireox, it's similar with other browsers:
It also tells you there how to set the default 'mail client' for the computer system to use.
Basically- Settings>Apps>Default apps>email
This is what will be used by the system, for instance if/when you right click a file and 'Send To > mail recipient'.
It will also be the default mail client by other apps, Word, Exel, etc. (Unless you override it in the settings of that particular app).