Remove an invalid email address from Adobe Sign

Are you using Adobe Sign via the adobe Sign webportal? In which case you can use the X next to the email to get rid of it:

If using Sign through Acrobat, then you'll need to dive into the registry and modiy/delete the following key

HKEY_CURRENT_USER\SOFTWARE\Adobe\Adobe Acrobat\DC\Workflows\cServices\cSendForSignature\cUser\taddressList

If using Reader, replace Acrobat with Acrobat Reader in the above path.