Outlook 2013, 2016 and 2019 do not delete inline attachments properly

Question

I have a user who forwarded an email thread to an external recipient.

Prior to sending she deleted all content below the new message, i.e. all the earlier replies/forwards which showed internal correspondence. Included in this deleted content was an inline embedded screenshot image. She kept the two existing attachments (which were listed as attached files NOT an inline/embedded attachments).

The recipient however received the email with all 3 attachments - the 2 that she meant to sent, plus the inline image that she had deleted, which contained sensitive information. The deleted inline image was converted to an attached file.

Long story short, deleting inline attachments is a risky business with Outlook/Exchange as it cannot be trusted. What you see is not what the recipient gets.

Further information:

1. We send emails in HTML format.
2. I can repeat this at will using the email thread at hand. Over and over, every time, as long as I use the original email thread. With the proper technical support therefore it should be possible to diagnose and fix this issue.
3. When I forward to an internal address (also using Outlook), the inline attachment is not visible to the recipient. However if this is then forwarded again externally to a gmail account, the inline attachment appears at the gmail inbox. This means the 'invisible attachment' is retained even through internal forwards after deletion.
4. This occurs when sending with Outlook 2013, Outlook 2016 and Outlook 2019 (not 365) with on-premises Exchange 2013. Outlook is up to date. Computers are all Win10/2004.
5. The problem is intermittent in that I cannot create a new email with an inline image, forward it, forward it again but delete the inline image. In most cases it works as expected; at the moment I only have the one thread that gets the inline image 'invisibly stuck' in it. This appears to be some form of corruption in the email thread.
6. I have searched online extensively and found 2 similar threads:
7. Same issue but unresolved: https://answers.microsoft.com/en-us/msoffice/forum/all/outlook-mail-deleted-attachment-can-be-seen-when/dffff517-9156-4338-873b-523e0bc5aebc
8. Similar but with subtle differences (we are NOT converting to text before sending), and applies to Exchange 2016 not 2013: https://support.microsoft.com/en-us/help/4015662/deleted-inline-picture-is-displayed-as-an-attachment-after-you-switch

This issue represents significant risk to our business. We cannot have clients and business partners receiving content that we chose not to send. I certainly hope that Microsoft will do something about this.

I am willing to provide further information and/or take additional diagnostic steps as required to assist in resolving this.

Cross-posting as recommended by moderator from https://answers.microsoft.com/en-us/msoffice/forum/msoffice_outlook-mso_win10-mso_2019/outlook-2013-2016-and-2019-do-not-delete-inline/81d04132-b2f2-4d2d-ba6f-a8b60a06309a?tm=1596755384008

Answer 1

I can repeat this at will using the email thread at hand. Over and over, every time, as long as I use the original email thread.
The problem is intermittent in that I cannot create a new email with an inline image, forward it, forward it again but delete the inline image.

According to your description, it seems that your issue is more related to this specific original email so far. Just the same as you mentioned, I really did a lot of tests in my environment but could not reproduce your issue as well.

In order to further confirm if your issue is related to your Outlook desktop client, it is suggested to do tests like below and see if the issue has any difference:

1. user A log in to his web mail and send the problematic original email thread to internal users B
2. user B also log in to the web mail, deleted all conten and forward this email to external userC
3. user C check this email in both Outlook desktop client and web mail.

Besides, there could be some add-ins in your Outlook client which may cause similar issue, so it is suggested to start your Outlook in safe mode (Press Win + R, type “outlook /safe”, press Enter.) to test and see if the issue continues.

Any update, please feel free to post back.