We have recently migrated our Mail service from an On-premise Exchange 2010 server to an Office 365 subscription. It's currently a hybrid solution but we plan on decommissioning the 2010 server in the next couple of weeks
It seems to be going Ok, except for some reason a few of our users have not been receiving emails that have attachments. It's odd because as far as I am aware there is no difference between the accounts having issues and those without issues. This only started happening after the mailboxes were migrated to O365.
At first I thought it was an attachment size limit so I went into the admin center and changed everyone's size limit to the maximum allowed value (153600 KB). That didn't seem to change anything.
Then I figured it might have something to do with our email filter ( We use Barracuda ), but then I learned that emails from our on premises Printer/Scanners were also getting lost for some users.
I had them look for the missing emails in the OWA but it wasn't there either, so It's probably not an issue with the Outlook program itself.
I've reinstalled O365 for the affected users but there was no improvement
This issue seems to only affect a handful of users and I'm not sure what could be causing the issue.
Any help would be appreciated.
Thanks.
Edit: Had a look through the message trace for the affected users. I can see messages from the same senders being delivered with no problem. But when I look for the messages from those senders with the attachments, there's nothing to be found. Is there some component on the old exchange server we're missing?
We discovered what the problem was. We had set up a forwarding rule on our barracuda that forwarded mail to our on-prem exchange server first, which then forwarded the mail to O365. Someone set this up months ago when we were first setting up the hybrid configuration and forgot about it. So any mail flow settings on the on-prem were still being applied. We changed the forwarding rule to send mail directly to O365 and now everything works just fine.