Now that we know you are using Chrome, my guess is that the form Autofill is storing the information. There seem to be bugs in the ability to edit this information using the settings, but this method should work:
- When using the Chrome browser, on the Fastmail Log in to your account screen click the Email field and use your keyboard up/down buttons to highlight an entry you wish to remove.
- Use Shift-Delete to remove that entry.
Bill