For which product you are looking for this use case, however, I found one document on communities related to your use case
We are using BMC Helix Remedyforce and we use Outlook 2016/Office365 for emails. The email template in question was created as a text type Template.
We wanted the email sent out for ticket approval to stand out more by using the "High Importance" tag. This is add Red exclamation mark when users receive email in their Inbox.
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Sending High Priority approval emails is not possible. There is similar idea on Salesforce.
However you can send high priority emails for regular conversation with clients from Outlook and add ticket number so email will get attached to ticket.