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Some emails not getting to the Teams channel

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Richard Hauer
Occasional Contributor

I have a Teams channel setup to accept emails from anyone (as in, there are no sending domain filters set).

 

If I email this channel directly from my Office 365 or GMail accounts, the messages are displayed.  However when I route the email from a smart-host (it's a custom setup, not an off the shelf package) then the message doesn't get through.

 

I can setup the smart host to send to my Office 365 mailbox address and the Teams address at the same time - I get the message but Teams does not.

 

Does anyone know if Teams is checking DKIM signing on the message, or if there's a SPAM filter of some kind? My smart host does not (yet) apply DKIM signing to the messages it routes, so I'm wondering if that's the issue.

 

The SPF records are set properly but the message content I've been using has been "test" this and "sample" that, which can end up in Junk sometimes.

 

13 Replies
Hi ,

It is unlikely that DKIM is being checking unless it is enabled within the Exchange Admin Centre.

Would recommend

1.) Add Teams Email Address as External Contact in Exchange Online
2.) Optional - If you want the email to be sent to you and the external contact add both to a distribution list in EAC

That should solve it!

Hope it answers your question!

Best, Chris
Don't think that's his end goal :P.

There is most likely some form of basic filtering on the e-mail addresses to channels. Are you spoofing the from/reply addresses coming in from that address? You said you get to your regular mailbox so that might not be it..

Can you access telnet or the logs for the message and make sure it's getting the 250ok sent to the server? I'd start there.
My point was that the smart host is either greylisting the maessages, there are rules on it or is not set to accept on/send to the address/domain of the Team, which is different than the custom domain - so by adding it as part of a DL the smart host would accept it being on the custom domain.

;D

Best, Chris

Thanks @Christopher Hoard - that's not quite what I meant.

 

Firstly, the "smarthost" is of my own making and isn't greylisting anything.  It's a mail router that you can send email to and will forward that email on to another mailbox for you.  I have setup a rule that the target for the router is a Teams email address (and an Outlook mailbox - I can have more than one target).  The Outlook mailbox receives the message, so the router is obviously working, but the Teams channel never does. This means the Teams mail feature must be rejecting the message.

 

I noticed that the router does not strip out the original DKIM headers when the mail comes in.  This means the message would definitely fail DKIM validation.  That's enough to send the message to the Outlook spam folder... I wonder if it's enough to also prevent the message from getting through to Teams.

 

I have a new build of the router that I'm testing that resets the DKIM headers properly - we'll see if this solves it!

Thanks Chris. That's pretty much my line of thinking too - I don't have easy access to the mail routers logs (because it doesn't log anything at present) so I was just thinking out loud to some extent. As I mentioned to Chris Hoard, the smart host (router) receives a properly signed message but did not strip out the existing DKIM headers, so any DKIM validation would definitely fail. Outlook was marking these messages as Spam, but still delivering them. I suspect Teams was just rejecting them outright.

I have an updated build to test on Monday that signs messages properly so we'll see what happens with that.

@Chris Webb So we updated the message router so it now properly signs messages with DKIM; I also have the system emailing me copies of any messages that come through.  So I **know** the messages are being sent on to the Teams channel, but they never appear in the list.

 

Is there any way to debug this? Messages sent to Teams are not ever getting into the channel which is a serious problem for us right now.

Did you ever try doing a manual Telnet type session to the teams server? You should have some form of sending server logs to see what response you are getting from the teams service.

@Chris Webb We are on Office 365 so not really sure if telneting in is plausible

But you said you use a smart host, you should be able to look into that and see logs or bounce e-mail off it etc.
Quite right.

The smart host thinks the message was delivered. So it seems to be getting filtered out by the Teams/SharePoint platform itself. This is why I suspected some kind of anti-spam system was at play.
Yeah, I'd create a ticket if you haven't already. I'm surprised it doesn't reject at the start, but def. if other domains are getting through it has to be blocking that smart host for whatever reason. Most likely due to spoofing. Not sure if you can pass or send e-mail from some other random domain through your smart host to see if it works. That's why I mention telnet cause you can just pick a random domain to use mail from: when using it.
Some messages sent directly get through. Oddly, some do not. More oddly than that, some of the messages going through the smart host get through and others don’t!

I could spoof the EHLO host but that would fail some anti-spam rules so I feel like setting it up correctly with DKIM and SPF in place gives it the best chance of success.

I checked the Forefront headers and the spam scores are 0 or close to 0. Plus messages from the smart host get through to Outlook on Office 365 just fine.

It’s a prickly problem with no logging at all sadly.
I mean, if you only use a few channels for this and it's not like a widely used thing, you might could look into using Flow, to monitor a mailbox and post messages via a webhook connector (for alerts). But not sure the number and flow in this situation but that could circumvent the whole e-mail engine.