When outbound filtering is enabled with Office 365, automatic reply messages(e.g. vacation leave message) from Office 365 is being rejected.
There is no issue when sending a normal outbound email. You can send to an external email address with outbound filtering enabled. The issue only happens for automatic reply messages from Office 365.
As part of sanity checking for outbound mails, HES rejects mails with null senders since this can be an indication of spam. This is a global practice. Out of office replies have null senders (RFC2298).