ATT00001.htm attachment being added to email

If you are sending an email with an attachment (images, documents, anything) and the person getting your message has an additional attachment called “ATT00001.htm” (or any derivative of the numbers ending with a “.htm” suffix) you might be wondering why this is happening?

What causes it?

This can happen if you tried to send an email that intermingles text sections and attachment sections. This is not isolated to a specific mail program and can happen with any mail program, 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. The Exchange mail 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”).

Solution & Workarounds

There is no way to make the Exchange mail server stop turning text sections into attachment sections. Our recommendation is that if you are using a Microsoft Exchange based email system 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 known workarounds 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.