It could have also been a simple mistake. Several Otto employees came from Waymo. Not hard to send email to John Smith <jsmith@google.com> instead of John Smith <jsmith@otto.com> by mistake if you were in regular contact with that person as part of their role at a previous job doing largely the same thing.
one of my bosses had my personal email and my work email on his auto complete. for some reason, he kept emailing me work stuff to my personal email because autocomplete recommended my personal email first (and he never bothered to check).
i had to ask him more than a couple of times to delete my personal email from his agenda so this wouldn't happen again.
I like autocomplete email addresses but I do worry that the day will come when I send something embarrassing or damaging to the wrong person. I never have afaik although I have sent a couple emails that resulted in a frantic follow up to please don't read the prior message.
Google corporate accounts might behave the same as gmail accounts. When an employee leaves, the account is disabled, and all emails bounce.
Hence, if jsmith left google, emails to jsmith@google.com would bounce, and bounce messages from google are returned as SMTP errors before the mail server accepts the contents of the message.
