My mom likes to keep tabs on my travels (mostly because she is a mom), so she's always asking me to forward my flight itineraries. While I usually remember to do this, I occasionally forget. Instead of dealing with a stressed-out mother who's convinced I've fallen into the middle of the Pacific Ocean because she can't track my flight, I decided to just set up a filter that automatically forwards all my travel messages to her.
First, you'll need to add forwarding addresses. You can only add forwarding addresses if the recipient gives permission, so you will probably only want to do this with people you're close to (your significant other, your mother) or with email addresses you own. To add a forwarding address, click the cog button, go to Settings and click the Forwarding and POP/IMAP tab. Under Forwarding, click "Add a forwarding address." Enter the email address you want to add and click Next; a confirmation email with a verification code will be sent to that email address. You or your recipient will need to click the link in that confirmation email before any automatic forwarding will take place.
Once you've added your forwarding addresses, you can create a filter by going back to Settings and clicking the Filters and Blocked Addresses tab. At the bottom of this screen, click "Create a new filter" and fill out the parameters -- I frequently fly Singapore Air, so I have created a filter for emails from email@example.com. Click "Create a filter with this search." On the next screen, check the box next to "Forward it" and select the forwarding addresses you want these emails to go to.