The below error can occur when the email you are sending to may have an access block. You need to ensure it matches what is in the Company Details and Payslip Settings.
To resolve this, confirm the username is correct in email payslip settings by going to Payroll Run > Email Payslips.
The username will show as User under Authentication on the left.
Go to Parameters > Company Details to check they match.
If they match, go back to Email Payslips and Generate Payslips. Update them and repeat this step if they do not.