Don't just forward the e-mail thread -- catch someone up on:

• The context
• The history
• The "angle" (if there is one)
• What you need from them to move things forward

"the biggest myth about communication is that it has taken place". (Or something like that)

Based on a recent chat I had with someone. They'd sent me an email asking for an update. But the email had a long thread that I wasn't familiar with in the context that I didn't have the full picture for.

Important to provide context, especially to people who are involved in lots of contexts.

Important context includes:

If there are implications for your group or org
A summary of the situation to date
An understanding of key players.
What is needed from the person and by when.
Examples:

"Reminder - this customer is looking for an update. We haven't given them a response in x weeks and soon there will be x/y/z implications. I asked x about this but they said to escalate to you because y."
"I'd appreciate it if you could weigh in on these options. Per the thread below, it looks like our choices are A vs B. I'll summarize the arguments for each in brief below but the full thread is there too. This decision impacts x. Could you weigh in within the next week?"
Etc. etc.