Problem

When replying to or forwarding a plain-text email in Outlook, you may be unable to insert or change a Signature 365 signature. This may be more noticeable when sending from a shared mailbox.

The signature is available in the Signature 365 add-in, but the following message is displayed in the task pane, and it cannot be selected.

The signature is not inserted. Signature insertion continues to work for HTML emails.

Cause

This is a limitation of how Outlook handles plain text messages.

In an HTML email, Outlook includes an internal marker that identifies whether a signature has been inserted and where it is located. Outlook does not retain this information in a plain-text email.

Without this marker, Signature 365 cannot reliably locate or replace an existing signature. Attempting to do so could place the signature incorrectly or insert multiple signatures. Signature 365 therefore intentionally prevents signatures from being changed in plain text emails.

Automatic signature insertion when initially composing a plain text message is supported. Changing the signature after it has been inserted is not supported.

Resolution

Change the Outlook message format from plain text to HTML, then insert or change the signature using the Signature 365 add-in.

In classic Outlook for Windows:

1. Open the message.

2. Select the Format Text tab.

3. Select HTML.

4. Insert the required signature from the Signature 365 add-in.