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On many (but not all) of my incoming email messages, I see a message saying:

This email is from an external mail server, be judicious when opening attachments or link

My mail server is - unfortunately - outlook.office365.com . I'm thinking it might be the culprit rather than my correspondents themselves, although I can't quite tell.

Anyway, this is super annoying, and basically meaningless. I mean, of course most email comes from an "external mail server" - not everyone on the planet uses the same server after all.

My question:

  1. Why am I even getting these notices?
  2. Other than "scrub" them ex-post-facto from the displayed message (e.g. with a Thunderbird extension) - is there something I can do to avoid getting them?
  • This is configured by an Administrator on the Exchange side. Microsoft Exchange has the ability to generate this particular message when it detects an email outside of your organization. Are you that Administrator? – Ramhound Aug 22 '20 at 9:51
  • I have already answered this question here – Ramhound Aug 22 '20 at 9:52
  • @Ramhound: I'm about the opposite thing of an administrator... :-( – einpoklum Aug 22 '20 at 9:55
  • I don't have the required environment to provide detailed screenshots, but as you can see from my answer to the potential duplicate, the message is generated and configured within the Admin Portal, which means it can only be changed by an Administrator. – Ramhound Aug 22 '20 at 9:58
  • Is your Office 365 account part of an organisation or a personal account? – Mokubai Aug 22 '20 at 12:48
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I believe that this message is coming from the sender's side and there is nothing you can do except scrubbing it out on your side.

It seems to me that the sender is situated within a corporate email system, but has decided to configure his own email server which is external to the corporate network, rather than using the corporate server. The corporate email software has then added this warning to each email sent by that user.

You may verify my theory by making a list of the sources of these messages, to see if they have a common source(s).

You may see examples for such configurations for Broadcom and Plesk (although I don't know if their servers add such a warning message).

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If it is outlook that is the reason, someone (some admin in your organisation?) put it there. It should not be the default, though defaults might have changed since I last looked.

If you have admin app access in your 365 account,

  • select admin app
  • admin centers -> exchange
  • select mail flow

There you should be able to alter the rules; if it's Outlook giving you this message, there should be a rule for it there.

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