ATT00001.htm attachment being added to email

If you are sending an email to someone and they are receiving it with attachments like “ATT00001.htm” (or any derivative of the numbers ending with a “.htm” suffix) that you didn’t attach, you might be wondering why this is happening.

What causes it?

This can happen if you tried to send an email with an attachment, and the system that you wrote the email in intermingles text sections and attachment sections. This is not isolated to a specific email client and can happen with any email client, however it is very often seen when a sender is using the Apple Mail application (including on iPhones and iPads) as the email client and they are using a Microsoft Exchange based email system to send those emails. An Exchange or exchange based email server insists that message text must always be first and attachments must always be last. As soon as the Exchange software sees one attachment in a message, it stops looking for text, and treats anything else in that message as an attachment. Any remaining text sections are converted into attachment sections, and given fake file names (like “ATT00001.htm”).

There is no way to make the Exchange mail server stop turning text sections into attachment sections; this is an email standard that these systems adhere to, that some other email systems ignore.

Solution & Workarounds

• If you are attaching photos, documents, or videos to an email (or your email contains photos or other images in-line) you can attempt to first put them into a ZIP file and then send that ZIP file instead. Even if it is just one attachment you are trying to send, you can put that single attachment into a ZIP file and send that. Many email clients will treat a ZIP file differently than any other attachment when sending the message over to the email server to be sent.

• If you are using a Microsoft Exchange based email system, it is highly recommended that you use the Outlook email application as your mail application. Outlook is included in Microsoft Office for Mac software for Apple desktops and laptops. The Outlook mobile app is available for free in the Apple App Store and Google Play Store for mobile devices.

There are some other workarounds that you may find on the internet that do things to attempt to include attachments at the end of a message, after text and after any signatures, however they are a 50/50 shot and generally will get removed anytime the mail app is updated; therefore we do not recommend them.