I often find certain portions of email more important than the rest, so is there a way for me to highlight or mark certain sentences for future reference?
Are there any mail clients that support something like this?
Microsoft Outlook lets you open a message and then edit it. You can then apply any formatting operation to it (including highlighting if the email is in HTML/Rich Text) before closing it.
Open the message into it's own window, then Edit -> Edit Message, and bring up the formatting toolbar.
The other option is to just forward the email to yourself, and edit it with highlighting before you send it.
none has the ability (which is a feature long asked for by power email users).
My current setup involves Thunderbird with the XNote add-on, and instead of highlighting, I use the notes to indicate important portions of my emails.
You could try that - it's more versatile too.