Highlight of text in an email

Dear all,
sorry I am new and this is my first post. I am also new to DTP.
I have imported an email from Mail. Now I want to highlight text and mark the important things in the mail. But the highlight function remains grey, I am unable to use it.
What is the trick please? Do I have to convert the email to standard text?
Thanks for your advise.
Kind regards

Emails aren’t editable, you have to convert them to rich text (see menu Data > Convert)

Thank you for your reply.
Do you recommend to convert them in RTF and delete the original email from DTP (but not the original email in my Mail program)?
Thanks for the advise.

Whether to delete the originally imported file is a personal choice. There isn’t a right or wrong answer here.