Outlook 2016 Message-ID oddity/inconsistency?

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I use Outlook 2003 with internet-mail only, namely a few Yahoo accounts, via POP3. I recently discovered that, for any email I send from Outlook (via Yahoo SMTP servers), the resulting unique message-id in the header ends in "…@[my local computer name].” I had expected it would end in "…@yahoo.com."

Now, my brother uses Outlook 2016 with a similar setup, namely a few Yahoo accounts, via POP3. Today I checked, and the header message-id for his sent mail does end in "...@yahoo.com" instead.

Is this difference to be expected simply because of Outlook 2003 vs 2016? If not, do you have any idea why my message headers would reveal my local computer name? And, in addition to a bit of privacy concerns, could this make my sent mail more likely to sometimes end up tagged as spam?

No ideas? If there are any Outlook internet mail users out there (IMAP or POP3), could you just check the value for "message-id" in the header on any sent email and report back as to whether it has a domain or a local computer name after the "@"? Thanks!
My hotmail-sent emails have dffgf4545g4.namprd88.prod.outlook.com at the end.

You might get more responses if you tell people how you are viewing the message id in the header.
Thanks for the reply. Good point about explaining how to view the Message-ID in the header. In fact, it led me to discover you can't see Message-ID's, or any other headers, for Sent Mail in Outlook. Instead, the way to view headers for an email you sent is by bcc-ing yourself on it and then looking at it in your Inbox copy (or just send yourself an email just to test). (For those who need it, at the end of this post I have pasted part of a Microsoft document on how to view headers in Outlook.) I find that "Message-ID:" is usually toward the bottom of the header, near "To:" and "Date:".

The Message-ID of the email I received notifying me of Vincenzo's post is shown by this line from the headers:
Message-ID: <65fef27e150cebcc44ceb8f428e060a3@forums.slipstick.com>

That is the kind of Message-ID format I would expect, namely a long alphanumeric followed followed by "@" and then a domain. But again, I discovered that the Message-ID for emails I send from Outlook via Yahoo servers is instead a long alphanumeric followed by "@[my local computer name]" (instead of something like "@yahoo.com" or "@mail.yahoo.com"). Again, that is the reason for my question. It seems odd that my local computer name is being divulged (not that it's particularly worrisome in my particular case, because I don't think it would be meaningful or useful to anyone). But I'd still appreciate knowing if anyone else is seeing their local-computer name after the "@", instead of a domain. And any theories as to why that's what I'm seeing.


From: View internet message headers in Outlook

View internet message headers in Outlook​

Outlook for Microsoft 365, Outlook 2019, Outlook 2016, Outlook 2013, Outlook 2010, Outlook 2007

  • Double-click an email message to open it outside of the Reading Pane.
  • Click File > Properties.
  • Header information appears in the Internet headers box.
    Tip: You can highlight the information in that box, press Ctrl+C to copy, and paste it into Notepad or Word to see the entire header at once.

(For Outlook 2003, you can see the headers box by opening the message and choosing View, then Options).
I just realized my reply above was incomplete. It should have said that the address shown was "at the end of the message ID."

Which you probably realized I meant.

Anyway, I have tried to reproduce the odd message id you are seeing and cannot make it happen.

You seem to imply that it only happens when sending from Yahoo accounts? Are you just sending from Yahoo to Yahoo account? Can you vary from that?
And what if you send from the Yahoo web interface?

Have you tried setting up another type of account in your Outlook and sending from it to see if it still happens?
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My Yahoo emails are sent from Outlook to a wide variety of domains.

Good suggestion about testing a send from the Yahoo web interface. When I do that, the resulting Message-ID ends in "@mail.yahoo.com." (Not surprisingly, this is true whether the header is viewed in Outlook or on Yahoo's web interface.) This is, again, the kind of thing I would expect to see even when the email is sent via POP3 from Outlook instead.

I should also note that, while I haven't used it in awhile, I also have a Comcast email address and POP3 account in Outlook, sharing the same pst as the Yahoo accounts. Since I often bcc myself on emails I still have a lot of the emails that I sent via the Comcast SMTP server, and the Message-ID's for those also show my local computer after the "@". So it doesn't happen only with my Yahoo accounts.

Just as a reminder, I am using Outlook 2003. I wonder if Message-ID's might have been constructed differently in that version?
I think you are right, it is an Outlook 2003 issue. Here is an article from 16 years ago

Thank you for that! I note that it also specifically discusses one concern I had (admittedly minor, since these are not business emails) that the domain-lacking, computer-name-exposing, Message-ID's on my emails might increase the likelihood that my emails end up flagged as spam.

In fact, my question originated during an email conversation I was having with a company's customer service, when my latest reply to them was returned by MAILER-DAEMON, and I noticed that the (to me mostly mumbo-jumbo) text showed my local computer name. (A couple other oddities were that (1) it also divulged what was apparently a personal email addresss (so-and-so@icloud.com) that was actually receiving the emails I was sending to customerservice@companyname.com; and (2) IIRC (and maybe I don't) it said something about violating *Yahoo's* rules.) Maybe this also explains why on rare occasion I have found a gone-missing bcc (as usual sent from Outlook 2003) in my Yahoo WebMail spam folder for that address.
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