Rogue Default Email Address

I must have done something wrong at some point, but I can’t work out what…

When I use the “Send Email” function, it always populates the sender and sender email address with the same information – my name and an email address of mine which is neither the information for the WordPress site admin, nor the logged-in user (I know you can click the link to make this happen), nor the default email address given in the email set-up, nor even an address I have used to register with the site! Now I’m working with Mailersend, I have to use the right domain, and this “default” one is a different one.

How do I change the default, please? (I can change it every single time, but that seems a waste of time and effort – and today, I forgot and lost the whole email!)

Thanks,
Mark