The most common reason this is received is due to an email password change made in your GMail Account.  Have you change your email password recently? 

Below are a few things to check in the system as well:

In Gmail account setting user need to add email addresses which he/she like to use as “From” address in CROSStrax. Need to make sure “treat as an alias” is unchecked (second screenshot).

You may need to add all employees email address whose email address ending with same domain name and want to use from address of user.