For as long as emails have been around, there have been 3 different email messaging formats: HTML, Rich Text, and Plain Text.

When you use an HTML email signature (one that includes images such as logos or profile pictures), you must use the HTML or Rich Text messaging format when sending out emails.

If you don’t, the pictures in your HTML email signature get added as attachments to your email. This happens because the plain text messaging format doesn’t support images, so instead of completely removing the images from your email, they are simply added as attachments.

How to Change the Messaging Format When Composing New Emails

Outlook 2010 onwards

This setting is under File > Options > Mail > Compose Messages > From the “Compose messages in this format” section, choose HTML.

This setting is under Tools > Options > Mail Format > From the “Compose messages in this format” section, choose HTML.

Apple Mail

With Apple Mail, when Rich Text is selected, it is automatically converted to HTML when sending emails.

This setting is under Mail > Preferences > Composing Tab > From the “Message Format” section, choose Rich Text.

How to Change the Messaging Format on Incoming Emails

What about when someone emails you in plain text / rich text format? In an ideal world, everyone will have their email clients set to compose new messages in HTML format, but unfortunately that is not the case.

If you receive an email in plain text / rich text format, you can easily change the message format, however it is a manual action you have to do for each message.

Outlook 2010 onwards

Open the message in a new window (double-click on the message) then click on Reply, Reply All or Forward. At the top, click on the Format Text tab, select HTML.

If all your incoming emails are being converted to plain text, check the setting below and make sure it is not ticked.

This setting is under File > Options > Trust Center > Trust Center Settings… > Email Security > Read as Plain Text > Read all standard mail in plain text.

Emails Converted to Plain Text by the Recipient

When you compose a new email in HTML format, the email doesn’t always stay in that format throughout the email chain. The email format can be changed by anyone who replies to that email.

For example, take a look at this example email trail to see how it can get converted between formats.

Debbie: Uses HTML email messaging format.

Harry: Uses Rich Text messaging format.

Tom: Uses Plain Text messaging format.

Debbie sends an email to Harry – Email is converted from HTML to Rich Text when it reaches Harry’s inbox.

Harry forwards that email to Tom – Email is converted from Rich Text to Plain Text when it reaches Tom’s inbox.

Tom forwards that email to Debbie – Email is delivered to Debbie’s mailbox as Plain Text.

But hang on, why is the email delivered to Debbie’s mailbox as Plain Text? Wouldn't it be converted to HTML format since Debbie’s email client uses HTML messaging format?

The answer is no. Email clients will automatically downgrade messaging formats, but in order to upgrade, you need to do it manually.

So, an email will freely convert from HTML > Rich Text > Plain Text, but once it’s in plain text format, someone will need to manually change it back to Rich Text or HTML by using the above steps.

Exchange Server Converting All Outgoing Messages to Plain Text

At times, your Microsoft Exchange Server can automatically convert all outgoing messages into plain text, and if that is the case, then you need to get in contact with your network administrator to remedy this problem.

However, this doesn’t normally happen, as the default Microsoft Exchange behavior is to send all messages in the format that the email client dictates.

If you’ve set your composing message format to be HTML, and your emails are still being converted to plain text and the recipient definitely hasn’t converted it, then your emails are most likely being converted to plain text by your Exchange server.

Images Are Embedded Instead of Linked

When creating your email signature, be sure to host your images on a web server and link to them from your email signature, instead of embedding them into the signature itself.

If you have an embedded image in your signature, most of the time Outlook will automatically convert them to attachments.

Anti-Virus Software Scanning Email

If you’ve tried everything and your email signature images are still being attached, it could be your anti-virus software (precisely the email scanning component) causing this issue.

You should try disabling the feature that scans emails to see if it still happens.