Microsoft Office 365 pitfall – happened while preparing a customer transfer from his existing mailhoster to O365. The domain was authorized with Office 365 and the outlook client dns records where activated in the domain. But no mx records set yet!
Customer called because of missing emails. Turns out emails sent from other O365 mailcustomers to this customer (mail domain prepared but mx records not yet pointing to O365) are sent to his new mailbox on O365. This is because of an exchange setting to handle it’s own domains with priority – effectively ignoring the mx records.
Bad enough, if this fact were publicly known or info in bold letters on the administration page available. But it isn’t. Experienced exchange admins sure know about it, but not everyone knowing how mail transport according to RFCs works has to be an exchange expert. Thank you Microsoft!