In the email invalid cause field, it shows the invalid cause such as 550 <firstname.lastname@example.org> No such user here. However, the email address in this field does not match the email address that I have for the person. I was thinking that maybe the email address had been updated in SFDC or Marketo, but I cannot locate any record of the email address being updated in SFDC or the record's activity history. I know for some emails, they have an alias, so is it possible that the alias is being populated into the email invalid cause instead of the actual email I am sending to? Or why isn't the email change, if there was one, showing up in the record's activity history?
I think that might be the opposite: you have the alias in your database and when sending the email, it's forwarded to an address that no longer exists, which shows up in the error.