Mystery Of Some UserA Emails Being Sent to UserB As Well

I have across an issue recently where I spent quite some time to figure out the cause. Few emails destined to UserA were also being delivered to UserB. If it was a forwarding, all emails would get forwarded but in this case it was few emails only and it was random. I made sure that there was no Exchange Online and / or Outlook rule causing that.

As per the message tacking on Exchange Online, I saw the following and checked all spam filters but couldn’t find anything there either.

250 2.1.605 Spam filter added recipients (redirect/bcc);250 2.1.605 Spam filter added recipients (redirect/bcc);250 2.1.605 Spam filter added recipients (redirect/bcc);250 2.1.605 Spam filter added recipients (redirect/bcc)

The issue turned out to be Outbound spam filter policy on Exchange Online even though Exchange Online spam filtering wasn’t being used.

1. Log on to Office 365 as Global Admin and browse to https://protection.office.com/antispam
2. Edit Outbound Spam filter policy.
3. Check Notifications section and see if there is anyone listed there, in my case there was.