You are trying to send a test email from your campaign and you get this error: 501 Sender syntax error?
If you are trying to send the test from an employee email, and you have set the send from to a variable then the email test will not go through.
For a test to go through, it needs to send from an email that has an account manager and since employees do not have account managers then the test will not be sent and you will receive this error.
If you have set the default from address as a variable (see image below)
then check to see if the Email address you are sending the test to belongs to an employee. If it is does then change it, maybe to a personal gmail account just for testing purposes and try to send the test email again.
It looks like you are trying to test from an email address that belongs to an employee. Tests will not go through without a contact manager when you use this variable in Default from address (please see the image below). Employees of your company do not have an account manager and that is why it is showing this error.
You can try to send the test from another email address that does not belong to an employee, for example a Gmail account that you can test from and let me know if that works.