I am getting the following MySQL error when installing or upgrading: An error occurred while trying to connect to your MySQL database: Access denied for user 'xxxxx'@'localhost' - why?
Do you use CPanel to create your MySQL users? If so you may have forgot to assign that user to your Interspire Email Marketer database. Don't worry though - it's a common problem and it's more to do with the bad design of CPanel.
To assign the user after you create it, you'll need to choose the user from the dropdown in CPanel and also the database you want to assign it to. Select "all" permissions and click the "add" button.