I have an Outlook app on my android phone. This is set up to work in conjunction with my desktop 365 app and is NOT the MS Outlook webmail version. I've been using it to correspond with a colleague for some years. The colleague's address is in the format peter@company.com. I sent an email the other day and it bounced back stating the the email sent to peter.company@gmail.com could not be delivered as it was spam. I did to change the outgoing address (why would I?). It hasn't happened with any other addresses. Does anyone have any idea what might have happened please?