I thought that Sharepoint List columns not available in dynamic content would help me, but apparently Dynamic Content only works for SharePoint List Items in the Subject or Body fields in the Outlook - Send an Email option (see the UsersEmail dynamic content item in the first screenshot):

And does not work if you want a custom value in the To field (see no UsersEmail, only Created By or Modified By Email dynamic content):

Is there some way to get this? I have an ASP.NET webform that employees fill out and it writes the files to SharePoint using a generic account's credentials. The problem with that is it means the 'Created By' field shows the generic account rather than the person who submitted the form. I don't want rejection or success e-mails going to the generic account's email address... that's useless.
Instead, I want to be able to pick from the entire list of Dynamic Content (this seems to be another case of Microsoft applications trying to be "smart" and failing) and then if it's not an email address, let the mail server handle it as undeliverable. Microsoft Flow should not prevent me from attempting to send to a custom value, which in my case will always be a valid e-mail address.
Again, is there a way around this limitation?