Knowledge - Details

Microsoft Outlook shows HTM file attachment from Apple senders
Date Added: 5/11/2023

When you open an email with Microsoft Outlook, there is an unexpected file attachment with HTM extension.  The filename is usually something like "Unnamed.htm", "ATT00001.htm", or "Untitled attachment 00003.htm".  The body of the email shows the first portion of the sender's body content, and the HTM attachment contains the remainder of the sender's body content.

When Apple Mail sends an email with an attachment, such as a PDF file or an image file, it often inserts that attachment in the middle of the email's body content, with the remainder of the body content following.  You can see that by viewing the raw email with a text editor.  Microsoft email clients expect all attachments to be listed at the end of the email, not in the middle, so when it detects one attachment, it assumes that anything following is another attachment.  Thus, it will show the intended attachment(s) and one or more extra HTM attachments, each containing a portion of what should have been the email's body content.

This also happens on the Microsoft email server, which can cause its spam/phishing filter to detect that the email has a suspicious HTM file attachment, and possibly reject the email.  In that case, the sender may receive a rejection notice telling them that their email was detected as spam or that it had a disallowed file attachment type.

The Apple email sender has to change some settings on their email client software:
1) Edit > Attachments > Always Send Windows-Friendly Attachments
2) Edit > Attachments > Always Insert Attachments at the End of Message

Ultimately, it would be nice if Microsoft would make their interpreter more defensive to handle this unexpected format correctly.  And, Apple should simply move their attachments to the end of the email, both to improve compatibility, and to make the raw email easier to read with a text editor.

Back to List

Disclaimer: Everything on this website is written for my own use. I disclaim any guarantees that the procedures and advice listed here are accurate, safe, or beneficial for anyone else. If you attempt to follow any procedures or advice shared here, you do it at your own risk. Part of IT work is knowing how to recover from problems.